feat: extract distinct enum values (#532)

Author: tearfur

jsonschema-generator/src/main/java/com/github/victools/jsonschema/generator/impl/module/EnumModule.java

@@ -126,6 +126,7 @@ private static List<String> extractEnumValues(ResolvedType enumType, Function<En
         }
         return Stream.of(erasedType.getEnumConstants())
                 .map(enumConstant -> enumConstantToString.apply((Enum<?>) enumConstant))
+                .distinct()
                 .collect(Collectors.toList());
     }