Check for duplicated content happens without plugin input
Currently, due to #5567, a check is performed in add_content and the error is raised , plugins might want to deal with duplicates themselves at the finalization stage of the repo version creation.
For the cases when a user can't change much (e.g. sync from a remote, copy different versions of the same advisory from multiple repos), Pulp should try its best to solve issues itself without producing an error.
The check can be performed after finalize_repo_version is happened but before a repo version is marked as complete.