We had a solution that used azure functions provide a rest API, to help reduce the load on the sitecore content delivery servers.
We used AzureExtensions.Swashbuckle which makes it so easy to add swagger documentation to Azure functions.
Suddenly the swagger UI no longer worked and we got the “An item with the same key has already been added. Key: 400 ” exception.
After going through all the commits, one by one I could not see how the code changes could introduce this error?
Until I noticed that there where 2 ProducesResponseType attributes with the same status code (i.e. 400).
It was soo obvious when I noticed it, but when I look at C# code for changes I guess I ignore comments and attributes.
Therefore I hope by drawing attention to my stupid mistake I can help others that experience this issue, Alan