[BUG] [Elm] elmPrefixCustomTypeVariants

Created by: andys8

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

    "Answers": {
      "type": "object",
      "required": [
        "houseLiving",
        "liabilityLegalProtection",
        "pets"
      ],
      "properties": {
        "houseLiving": {
          "type": "string",
          "enum": [
            "RENTED",
            "OWN_HOUSE",
            "OWN_FLAT",
            "PARENTS_HOUSE",
            "SHARED_FLAT",
            "RENT_FREE"
          ]
        },
        "liabilityLegalProtection": {
          "type": "array",
          "items": { "type": "string", "enum": ["BUILDER", "LANDLORD"] }
        },
        "pets": {
          "type": "array",
          "items": {
            "type": "string",
            "enum": ["DOG", "CAT", "HORSE", "AQUARIUM"]
          }
        }
      },
      "title": "Answers"
    }

type HouseLiving
    = HouseLivingRENTED
    | HouseLivingOWNHOUSE
    | HouseLivingOWNFLAT
    | HouseLivingPARENTSHOUSE
    | HouseLivingSHAREDFLAT
    | HouseLivingRENTFREE


type LiabilityLegalProtection
    = LiabilityLegalProtectionLiabilityLegalProtectionLiabilityLegalProtectionLiabilityLegalProtectionBUILDER
    | LiabilityLegalProtectionLiabilityLegalProtectionLiabilityLegalProtectionLiabilityLegalProtectionLANDLORD

type Pets
    = PetsPetsPetsPetsDOG
    | PetsPetsPetsPetsCAT
    | PetsPetsPetsPetsHORSE
    | PetsPetsPetsPetsAQUARIUM

The actual output for LiabilityLegalProtection and Pets should only add the prefix a single time.

openapi-generator version

v4.0.0-beta3

Command line used for generation

    docker run --rm \
                -v "$PWD:/local" \
                openapitools/openapi-generator-cli:${OPEN_API_VERSION} generate \
                    -i /local/swagger.json \
                    -o "/local/$SDK_DIR/$generator" \
                    -g "$generator" \
                    --invoker-package "crm" \
                    --additional-properties elmEnableCustomBasePaths=true \
                    --additional-properties elmPrefixCustomTypeVariants=true

Suggest a fix

https://github.com/OpenAPITools/openapi-generator/blob/69ad9a9957fc0dca9dbd792cb9a6a3b6f5bf197c/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ElmClientCodegen.java#L696-L698

A fix would be to check if the prefix is already part of the value. This will work, but will not remove the root cause. It seems that certain cases leed to concatenating the prefix 4 times. In other cases it's working fine.

@eriktim What do you think?