Forbid mutliple @ApiResponse annotations with the same response code

[jochenberger]

The response code it the key in a map, e.g.

{
	"responses": {
		"200": {
			"description": "Person loaded",
			"content": ...
		},
		"404": {
			"description": "Person not found",
			"content": ...
		}
	}
}

See also https://swagger.io/docs/specification/v3_0/describing-responses/

So, if a method is annotated with multiple @ApiResponse annotations that define the same responseCode, it is not clear what should happen. It would be nice if that was prevented by the library.

[jochenberger]

See also #3379