[BUG][CSHARP][generichost] Many-to-one discriminator mapping emits duplicate local variables (CS0128) #25640

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists? (Reproduced on 7.26.0, 7.27.0 and master df804ac9802fba0c51a64f8c3cb03191ed3dde2d (openapitools/openapi-generator-cli:latest, 7.28.0-SNAPSHOT); library=restsharp builds on all three.)
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

When a oneOf discriminator mapping sends two or more values to the same schema, the csharp generichost library emits one local variable per mapping entry instead of one per distinct schema, all in the same scope of the converter's Read method. The generated client does not compile:

error CS0128: A local variable or function named 'cat' is already defined in this scope

A many-to-one mapping is valid: discriminator.mapping maps values to schemas and the specification does not require it to be injective. The restsharp library compiles the same spec without errors.

openapi-generator version

7.26.0 (CLI), 7.27.0 and master df804ac980 (7.28.0-SNAPSHOT), all with the same CS0128. JsonConverter.mustache changed after 7.27.0 (#25486), but the block that declares the variables ({{#mappedModels}} ... {{classname}}{{nrt?}} ... = null;) did not.

OpenAPI declaration file content

openapi: 3.0.3
info: {title: repro, version: "1"}
paths:
  /pet:
    get:
      operationId: getPet
      responses:
        "200":
          description: ok
          content:
            application/json:
              schema: {$ref: "#/components/schemas/Pet"}
components:
  schemas:
    Pet:
      oneOf:
        - $ref: "#/components/schemas/Cat"
        - $ref: "#/components/schemas/Dog"
      discriminator:
        propertyName: kind
        mapping:
          cat: "#/components/schemas/Cat"
          kitten: "#/components/schemas/Cat"
          dog: "#/components/schemas/Dog"
    Cat:
      type: object
      required: [kind]
      properties:
        kind: {type: string}
        meows: {type: boolean}
    Dog:
      type: object
      required: [kind]
      properties:
        kind: {type: string}
        barks: {type: boolean}

Generation Details

openapi-generator-cli generate -i spec.yaml -g csharp -o out \
  --additional-properties=targetFramework=net8.0,packageName=Repro
dotnet build out/src/Repro/Repro.csproj

Actual output

out/src/Repro/Model/Pet.cs, PetJsonConverter.Read:

Cat? cat = null;
Dog? dog = null;
Cat? cat = null;

The per-value branches further down are fine: kitten deserializes into the same cat variable as cat does, so only the declarations need deduplicating.

Expected output

One declaration per distinct mapped schema:

Cat? cat = null;
Dog? dog = null;

Steps to reproduce

1. Save the spec above as spec.yaml.
2. Run the generation command (default library, i.e. generichost).
3. dotnet build fails with CS0128 at the second Cat? cat = null;.
4. Add library=restsharp to --additional-properties and the build succeeds.

Related issues/PRs

Searched for "generichost discriminator", "CS0128" and "discriminator mapping same schema". #20059 and #22284 are about generichost discriminator mappings too, but they report a different symptom (mapping names ignored), not duplicate declarations.

Suggest a fix

Deduplicate mappedModels by model name before rendering the declaration block in csharp/libraries/generichost/JsonConverter.mustache, or declare the variables from composedSchemas.oneOf instead of mappedModels. The template already skips x-duplicated-data-type there, but that flag is not set for repeated mapping targets.