[BUG] allOf used to add constraints for string properties results in faulty generated DTO class if property is set multiple times

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

schemas with allOf definitions setting the same property multiple times result in a generated type with no property at all

openapi-generator version

I used commit hash 1b2917d69f13ea0545f755a6e2963923ff2fb367 and the "csharp" generator as well as a new generator for a different target language I'm developing right now.

OpenAPI declaration file content or url

{
    "openapi": "3.0.3",
    "info": {
      "title": "allOf used to add constraints bug",
      "description": "allOf used to add constraints for string properties results in faulty generated DTO class",      
      "version": "1b2917d69f13ea0545f755a6e2963923ff2fb367"
    },
    "paths": {
        "/actual": {
            "get": {
                "responses": {
                    "200": {
                        "description": "reproduces the bug",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/Actual"
                                }
                            }
                        }
                    }
                }                
            }
        },
        "/expected": {
            "get": {
                "responses": {
                    "200": {
                        "description": "returns expected type",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/Expected"
                                }
                            }
                        }
                    }
                }                
            }
        }
    },
    "components": {
        "schemas": {
            "Actual": {
                "properties": {
                    "myString": {
                        "allOf": [
                            {"type": "string"},
                            {"pattern": "[a-z]*"},
                            {"pattern": "[a-zA-Z]*"}
                        ]
                    }
                }
            },
            "Expected": {
                "properties": {
                    "myString": {
                        "type": "string",
                        "pattern": "[a-z]*"
                    }
                }
            }
        }
    }
}

Generation Details

My settings for generator as code:

        final CodegenConfigurator configurator = new CodegenConfigurator()
                .setGeneratorName("csharp")
                .setApiPackage("Rest")
                .setModelPackage("Rest.Dto")
                .setOpenAPINormalizer(Map.of("REFACTOR_ALLOF_WITH_PROPERTIES_ONLY", "true"))
                .setInputSpec("minimal_config_for_allOf_bug.json")
                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));

        // when
        final ClientOptInput clientOptInput = configurator.toClientOptInput();
        DefaultGenerator generator = new DefaultGenerator();
        Map<String, File> files = generator.opts(clientOptInput).generate().stream().collect(Collectors.toMap(File::getName, Function.identity()));

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/10010

Suggest a fix

In case of allOf properties of the same property should be merged (taking the first or the last if set multiple times, the spec doesn't state anything for that case), that way there would be no need to generate a new type, especially for basic type properties.

Start with the supplied minimal OpenAPI declaration and the Java CodegenConfigurator setup using the csharp generator and REFACTOR_ALLOF_WITH_PROPERTIES_ONLY. Reproduce the generated Actual and Expected DTOs, then trace allOf property handling; done means the repeated string constraints no longer remove myString from the generated type.