diff --git a/apps/guide/__tests__/GuideLessonFrontmatter_.test.res b/apps/guide/__tests__/GuideLessonFrontmatter_.test.res
new file mode 100644
index 000000000..a68607a34
--- /dev/null
+++ b/apps/guide/__tests__/GuideLessonFrontmatter_.test.res
@@ -0,0 +1,139 @@
+open Vitest
+
+let sourcePath = "test/lesson.mdx"
+
+let completeLesson = `---
+position: 1
+id: test-lesson
+missionLabel: Test mission
+title: Test lesson
+description: A test lesson.
+exercise:
+  id: test-lesson/example
+  title: Test exercise
+  initialCode: |
+    let answer = 42
+  expectedOutput: "42"
+---
+
+Test body.`
+
+let completeLessonResult = () =>
+  switch GuideLessonFrontmatter.parse(~raw=completeLesson, ~sourcePath) {
+  | Ok(lesson) => lesson
+  | Error(message) => throw(Failure(message))
+  }
+
+test("parses complete guide lesson frontmatter", async () => {
+  expect(GuideLessonFrontmatter.parse(~raw=completeLesson, ~sourcePath))->toEqual(
+    Ok({
+      GuideLesson.id: "test-lesson",
+      position: 1,
+      sourcePath,
+      missionLabel: "Test mission",
+      title: "Test lesson",
+      description: "A test lesson.",
+      content: "Test body.",
+      exercise: {
+        id: "test-lesson/example",
+        title: "Test exercise",
+        initialCode: "let answer = 42",
+        check: ExpectedOutput("42"),
+      },
+    }),
+  )
+})
+
+test("rejects guide lesson frontmatter without a lesson id", async () => {
+  let invalidLesson = completeLesson->String.replace("id: test-lesson\n", "")
+
+  expect(GuideLessonFrontmatter.parse(~raw=invalidLesson, ~sourcePath))->toEqual(
+    Error(`Guide lesson ${sourcePath} frontmatter "id" must be a non-empty string.`),
+  )
+})
+
+test("collects parsed guide lessons", async () => {
+  let firstLesson = completeLessonResult()
+  let secondLesson = {
+    ...firstLesson,
+    id: "second-lesson",
+    sourcePath: "test/second-lesson.mdx",
+    exercise: {...firstLesson.exercise, id: "second-lesson/example"},
+  }
+
+  expect(GuideLessonContent.collect([Ok(firstLesson), Ok(secondLesson)]))->toEqual(
+    Ok([firstLesson, secondLesson]),
+  )
+})
+
+test("preserves the first frontmatter error while collecting lessons", async () => {
+  let firstLesson = completeLessonResult()
+
+  expect(
+    GuideLessonContent.collect([Ok(firstLesson), Error("first error"), Error("second error")]),
+  )->toEqual(Error("first error"))
+})
+
+test("accepts a guide lesson collection with distinct identifiers and positions", async () => {
+  let firstLesson = completeLessonResult()
+  let secondLesson = {
+    ...firstLesson,
+    id: "second-lesson",
+    position: 2,
+    sourcePath: "test/second-lesson.mdx",
+    exercise: {...firstLesson.exercise, id: "second-lesson/example"},
+  }
+
+  expect(GuideLessonFrontmatter.validate([firstLesson, secondLesson]))->toEqual(Ok())
+})
+
+test("rejects duplicate guide lesson ids", async () => {
+  let firstLesson = completeLessonResult()
+  let secondLesson = {...firstLesson, sourcePath: "test/second-lesson.mdx"}
+
+  expect(GuideLessonFrontmatter.validate([firstLesson, secondLesson]))->toEqual(
+    Error(
+      GuideLessonFrontmatter.DuplicateLessonId({
+        value: "test-lesson",
+        sourcePaths: ["test/lesson.mdx", "test/second-lesson.mdx"],
+      }),
+    ),
+  )
+})
+
+test("rejects duplicate guide exercise ids", async () => {
+  let firstLesson = completeLessonResult()
+  let secondLesson = {
+    ...firstLesson,
+    id: "second-lesson",
+    sourcePath: "test/second-lesson.mdx",
+  }
+
+  expect(GuideLessonFrontmatter.validate([firstLesson, secondLesson]))->toEqual(
+    Error(
+      GuideLessonFrontmatter.DuplicateExerciseId({
+        value: "test-lesson/example",
+        sourcePaths: ["test/lesson.mdx", "test/second-lesson.mdx"],
+      }),
+    ),
+  )
+})
+
+test("rejects duplicate guide lesson positions", async () => {
+  let firstLesson = completeLessonResult()
+  let secondLesson = {
+    ...firstLesson,
+    id: "second-lesson",
+    sourcePath: "test/second-lesson.mdx",
+    exercise: {...firstLesson.exercise, id: "second-lesson/example"},
+  }
+
+  expect(GuideLessonFrontmatter.validate([firstLesson, secondLesson]))->toEqual(
+    Error(
+      GuideLessonFrontmatter.DuplicatePosition({
+        value: "1",
+        sourcePaths: ["test/lesson.mdx", "test/second-lesson.mdx"],
+      }),
+    ),
+  )
+})
diff --git a/apps/guide/app/GuideHomeRoute.res b/apps/guide/app/GuideHomeRoute.res
index 203547e4c..1f36b5179 100644
--- a/apps/guide/app/GuideHomeRoute.res
+++ b/apps/guide/app/GuideHomeRoute.res
@@ -3,9 +3,14 @@ type loaderData = {
   lessons: array<GuideLesson.t>,
 }
 
+exception LessonLoadError(string)
+
 let loader: ReactRouter.Loader.t<loaderData> = async _ => {
   let compilerData = await GuideCompilerData.load()
-  let lessons = GuideLessonContent.load()
+  let lessons = switch GuideLessonContent.load() {
+  | Ok(lessons) => lessons
+  | Error(message) => throw(LessonLoadError(message))
+  }
 
   {compilerData, lessons}
 }
diff --git a/apps/guide/app/GuideLessonContent.res b/apps/guide/app/GuideLessonContent.res
index 1e5d60e7c..7f64f1383 100644
--- a/apps/guide/app/GuideLessonContent.res
+++ b/apps/guide/app/GuideLessonContent.res
@@ -1,69 +1,22 @@
-let fail = message => JsExn.throw(Error(message))
-
-let fieldLabel = (~sourcePath, ~key) => `Guide lesson ${sourcePath} frontmatter "${key}"`
-
-let readString = (~dict, ~sourcePath, ~key) =>
-  switch dict->Dict.get(key) {
-  | Some(JSON.String(value)) if value->String.trim !== "" => value
-  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be a non-empty string.`)
-  }
-
-let readOptionalString = (~dict, ~sourcePath, ~key) =>
-  switch dict->Dict.get(key) {
-  | Some(JSON.String(value)) => Some(value)
-  | Some(_) => fail(`${fieldLabel(~sourcePath, ~key)} must be a string when present.`)
-  | None => None
-  }
-
-let readInt = (~dict, ~sourcePath, ~key) =>
-  switch dict->Dict.get(key) {
-  | Some(JSON.Number(value)) => value->Float.toInt
-  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be a number.`)
-  }
-
-let readObject = (~dict, ~sourcePath, ~key) =>
-  switch dict->Dict.get(key) {
-  | Some(JSON.Object(value)) => value
-  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be an object.`)
-  }
-
-let frontmatterObject = (~frontmatter, ~sourcePath) =>
-  switch frontmatter {
-  | JSON.Object(dict) => dict
-  | _ => fail(`Guide lesson ${sourcePath} must use object frontmatter.`)
-  }
-
-let exerciseFromFrontmatter = (~dict, ~sourcePath): GuideLesson.exercise => {
-  let check = switch readOptionalString(~dict, ~sourcePath, ~key="expectedOutput") {
-  | Some(expectedOutput) => GuideLesson.ExpectedOutput(expectedOutput)
-  | None => GuideLesson.Manual
-  }
-
-  {
-    id: readString(~dict, ~sourcePath, ~key="id"),
-    title: readString(~dict, ~sourcePath, ~key="title"),
-    initialCode: readString(~dict, ~sourcePath, ~key="initialCode")->String.trimEnd,
-    check,
-  }
-}
-
 let lessonFromFile = sourcePath => {
   let raw = Node.Fs.readFileSync(sourcePath)
-  let {frontmatter, content}: MarkdownParser.result = MarkdownParser.parseSync(raw)
-  let dict = frontmatterObject(~frontmatter, ~sourcePath)
-  let exerciseDict = readObject(~dict, ~sourcePath, ~key="exercise")
+  GuideLessonFrontmatter.parse(~raw, ~sourcePath)
+}
 
-  {
-    GuideLesson.id: readString(~dict, ~sourcePath, ~key="id"),
-    position: readInt(~dict, ~sourcePath, ~key="position"),
-    sourcePath,
-    missionLabel: readString(~dict, ~sourcePath, ~key="missionLabel"),
-    title: readString(~dict, ~sourcePath, ~key="title"),
-    description: readString(~dict, ~sourcePath, ~key="description"),
-    content: content->String.trim,
-    exercise: exerciseFromFrontmatter(~dict=exerciseDict, ~sourcePath),
+let collect = results =>
+  results->Array.reduce(Ok([]), (accumulator, result) =>
+    switch (accumulator, result) {
+    | (Ok(lessons), Ok(lesson)) => Ok([...lessons, lesson])
+    | (Error(message), _) => Error(message)
+    | (_, Error(message)) => Error(message)
+    }
+  )
+
+let validateAndSort = lessons =>
+  switch GuideLessonFrontmatter.validate(lessons) {
+  | Ok() => Ok(lessons->GuideLesson.sort)
+  | Error(error) => Error(GuideLessonFrontmatter.validationErrorMessage(error))
   }
-}
 
 let rec scanDir = currentDir =>
   Node.Fs.readdirSync(currentDir)->Array.flatMap(entry => {
@@ -80,4 +33,5 @@ let rec scanDir = currentDir =>
 
 let lessonsDir = () => Node.Path.join2(Node.Process.cwd(), "app/lessons")
 
-let load = (~dir=lessonsDir()) => scanDir(dir)->Array.map(lessonFromFile)->GuideLesson.sort
+let load = (~dir=lessonsDir()) =>
+  scanDir(dir)->Array.map(lessonFromFile)->collect->Result.flatMap(validateAndSort)
diff --git a/apps/guide/app/GuideLessonFrontmatter.res b/apps/guide/app/GuideLessonFrontmatter.res
new file mode 100644
index 000000000..06efece83
--- /dev/null
+++ b/apps/guide/app/GuideLessonFrontmatter.res
@@ -0,0 +1,147 @@
+type duplicate = {
+  value: string,
+  sourcePaths: array<string>,
+}
+
+type validationError =
+  | DuplicateLessonId(duplicate)
+  | DuplicateExerciseId(duplicate)
+  | DuplicatePosition(duplicate)
+
+type identifiedValue = {
+  value: string,
+  sourcePath: string,
+}
+
+exception InvalidFrontmatter(string)
+
+let fail = message => throw(InvalidFrontmatter(message))
+
+let fieldLabel = (~sourcePath, ~key) => `Guide lesson ${sourcePath} frontmatter "${key}"`
+
+let readString = (~dict, ~sourcePath, ~key) =>
+  switch dict->Dict.get(key) {
+  | Some(JSON.String(value)) if value->String.trim !== "" => value
+  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be a non-empty string.`)
+  }
+
+let readOptionalString = (~dict, ~sourcePath, ~key) =>
+  switch dict->Dict.get(key) {
+  | Some(JSON.String(value)) => Some(value)
+  | Some(_) => fail(`${fieldLabel(~sourcePath, ~key)} must be a string when present.`)
+  | None => None
+  }
+
+let readInt = (~dict, ~sourcePath, ~key) =>
+  switch dict->Dict.get(key) {
+  | Some(JSON.Number(value)) => value->Float.toInt
+  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be a number.`)
+  }
+
+let readObject = (~dict, ~sourcePath, ~key) =>
+  switch dict->Dict.get(key) {
+  | Some(JSON.Object(value)) => value
+  | _ => fail(`${fieldLabel(~sourcePath, ~key)} must be an object.`)
+  }
+
+let frontmatterObject = (~frontmatter, ~sourcePath) =>
+  switch frontmatter {
+  | JSON.Object(dict) => dict
+  | _ => fail(`Guide lesson ${sourcePath} must use object frontmatter.`)
+  }
+
+let exerciseFromFrontmatter = (~dict, ~sourcePath): GuideLesson.exercise => {
+  let check = switch readOptionalString(~dict, ~sourcePath, ~key="expectedOutput") {
+  | Some(expectedOutput) => GuideLesson.ExpectedOutput(expectedOutput)
+  | None => GuideLesson.Manual
+  }
+
+  {
+    id: readString(~dict, ~sourcePath, ~key="id"),
+    title: readString(~dict, ~sourcePath, ~key="title"),
+    initialCode: readString(~dict, ~sourcePath, ~key="initialCode")->String.trimEnd,
+    check,
+  }
+}
+
+let fromRaw = (~raw, ~sourcePath) => {
+  let {frontmatter, content}: MarkdownParser.result = MarkdownParser.parseSync(raw)
+  let dict = frontmatterObject(~frontmatter, ~sourcePath)
+  let exerciseDict = readObject(~dict, ~sourcePath, ~key="exercise")
+
+  {
+    GuideLesson.id: readString(~dict, ~sourcePath, ~key="id"),
+    position: readInt(~dict, ~sourcePath, ~key="position"),
+    sourcePath,
+    missionLabel: readString(~dict, ~sourcePath, ~key="missionLabel"),
+    title: readString(~dict, ~sourcePath, ~key="title"),
+    description: readString(~dict, ~sourcePath, ~key="description"),
+    content: content->String.trim,
+    exercise: exerciseFromFrontmatter(~dict=exerciseDict, ~sourcePath),
+  }
+}
+
+let parse = (~raw, ~sourcePath) =>
+  try {
+    Ok(fromRaw(~raw, ~sourcePath))
+  } catch {
+  | InvalidFrontmatter(message) => Error(message)
+  | JsExn(error) =>
+    Error(
+      error
+      ->JsExn.message
+      ->Option.getOr(`Guide lesson ${sourcePath} could not parse frontmatter.`),
+    )
+  }
+
+let findDuplicate = values =>
+  values->Array.findMap(current =>
+    values
+    ->Array.find(other => other.sourcePath !== current.sourcePath && other.value === current.value)
+    ->Option.map(other => {
+      value: current.value,
+      sourcePaths: [current.sourcePath, other.sourcePath],
+    })
+  )
+
+let lessonIds = (lessons: array<GuideLesson.t>) =>
+  lessons->Array.map(lesson => {value: lesson.id, sourcePath: lesson.sourcePath})
+
+let exerciseIds = (lessons: array<GuideLesson.t>) =>
+  lessons->Array.map(lesson => {value: lesson.exercise.id, sourcePath: lesson.sourcePath})
+
+let positions = (lessons: array<GuideLesson.t>) =>
+  lessons->Array.map(lesson => {
+    value: lesson.position->Int.toString,
+    sourcePath: lesson.sourcePath,
+  })
+
+let validate = lessons =>
+  switch lessons->lessonIds->findDuplicate {
+  | Some(duplicate) => Error(DuplicateLessonId(duplicate))
+  | None =>
+    switch lessons->exerciseIds->findDuplicate {
+    | Some(duplicate) => Error(DuplicateExerciseId(duplicate))
+    | None =>
+      switch lessons->positions->findDuplicate {
+      | Some(duplicate) => Error(DuplicatePosition(duplicate))
+      | None => Ok()
+      }
+    }
+  }
+
+let validationErrorMessage = error =>
+  switch error {
+  | DuplicateLessonId({value, sourcePaths}) =>
+    `Duplicate guide lesson id "${value}" in ${sourcePaths->Array.join(" and ")}.`
+  | DuplicateExerciseId({value, sourcePaths}) =>
+    `Duplicate guide exercise id "${value}" in ${sourcePaths->Array.join(" and ")}.`
+  | DuplicatePosition({value, sourcePaths}) =>
+    `Duplicate guide lesson position "${value}" in ${sourcePaths->Array.join(" and ")}.`
+  }
+
+let validateOrFail = lessons =>
+  switch lessons->validate {
+  | Ok() => lessons
+  | Error(error) => error->validationErrorMessage->fail
+  }