[REQ] Make sure enums are unique

Is your feature request related to a problem? Please describe.

I got a json with spec version 2 from the third party vendor. It contains enums in some places and some of them have duplications. Of course this is a bug in json and should be fixed by the vendor, but would be nice if the generator checked it and removed duplications before generating java enums

Describe the solution you'd like

Check and remove duplicates from enums before generating java enums. Print a warning about duplications found

Describe alternatives you've considered

Well, so far I wrote a gradle script which parses json and removes duplicates before running generator. It makes my gradle script big and ugly

gradle plugin configuration

openApiGenerate {
    generatorName = "java"
    inputSpec = "$buildDir/generated/json/my.json".toString()
    validateSpec = false
    outputDir = "$buildDir/generated/myclasses".toString()
    apiPackage = "foo"
    invokerPackage = "bar"
    modelPackage = "foobarbar"
    configOptions = [
            dateLibrary         : 'java8',
            java8               : 'true',
            library             : 'native',
            serializationLibrary: 'jackson',
            useRuntimeException : 'true',
            asyncNative         : 'true'
    ]
    globalProperties = [
            apiDocs  : 'false',
            modelDocs: 'false',
            apiTests: 'false',
            modelTests: 'false'
    ]

    sourceSets.main.java.srcDirs += "$buildDir/generated/myclasses/src/main/java"
}

gradle plugin version - id "org.openapi.generator" version "5.0.1"

Start from the Java generator entry point selected by generatorName and trace how enums from the inputSpec are processed before Java code is emitted. Inspect existing warning and generator-test patterns, then verify that duplicate enum values are removed and a warning is printed for each affected enum.