ImageBuilder should not allow duplicate OS/architecture under the same shared tags

Please see https://github.com/dotnet/dotnet-docker/issues/4634 for more details.

Essentially, ImageBuilder will allow you to have multiple OS types or architectures categorized under the same shared tags, which results in the readmes being incorrect. When the images are published to MAR/DH, pulling the tag will only give you the first image in the manifest with the duplicate architecture. This arrangement should result in an error at readme or Dockerfile generation time as it results in readmes being created with incorrect tag information for some images.