fix: Remove duplicate oneOf schemas during pre-processing

Author: ranger-ross

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -1009,6 +1009,14 @@ public void postProcessParameter(CodegenParameter parameter) {
     @Override
     @SuppressWarnings("unused")
     public void preprocessOpenAPI(OpenAPI openAPI) {
+        if (openAPI.getComponents() != null) {
+            // Remove duplicate oneOf
+            Map<String, Schema> schemas = new HashMap<>(openAPI.getComponents().getSchemas());
+            for (var schema : schemas.values()) {
+                ModelUtils.deduplicateOneOfSchema(schema);
+            }
+        }
+
         if (useOneOfInterfaces && openAPI.getComponents() != null) {
             // we process the openapi schema here to find oneOf schemas and create interface models for them
             Map<String, Schema> schemas = new HashMap<>(openAPI.getComponents().getSchemas());

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -2243,6 +2243,19 @@ public static Schema simplyOneOfAnyOfWithOnlyOneNonNullSubSchema(OpenAPI openAPI
         return schema;
     }
 
+    /**
+     * Removes duplicate `oneOf` from a given schema
+     *
+     * @param schema Schema
+     */
+    public static void deduplicateOneOfSchema(Schema<?> schema) {
+        if (schema.getOneOf() == null) {
+            return;
+        }
+        Set<Schema> deduplicated = new LinkedHashSet<>(schema.getOneOf());
+        schema.setOneOf(new ArrayList<>(deduplicated));
+    }
+
     /**
      * Check if the schema is of type 'null' or schema itself is pointing to null
      * <p>