[Bug]: Duplicated default headers in ApiClient for Java RestTemplate

Description

It should be possible to set default header for each HTTP request. Currently generated ApiClient class for resttemplate library does not allow that and creates duplicates.

The same issue was fixed 8 years ago in version 2 of the swagger-codegen: https://github.com/swagger-api/swagger-codegen/pull/6981 however the fix was never backported to the version 3.

For example, after executing:
apiClient.addDefaultHeader("TestHeader", "foo");
apiClient.addDefaultHeader("TestHeader", "bar");
Only TestHeader with value bar should be added to the request.

Version

all

Language / Generator Affected

Java - resttemplate library

Related Issues / Repos

https://github.com/swagger-api/swagger-codegen/issues/6976

Checklist

• I have searched the existing issues to avoid duplicates.
• I have included a minimal reproducible spec or configuration.
• I have clearly described the steps to reproduce the issue.
• I have specified which generator or template is affected.

Start by locating the generated ApiClient for the Java resttemplate library and compare its default-header behavior with the referenced swagger-codegen pull request #6981. Reproduce the two addDefaultHeader calls from the issue and verify that the generated request contains only the latest value for TestHeader.