What causes "Duplicates found Mirror may not be complete" when mirroring UVs

I have a mesh that I modeled one half of with mirror modify. I did things in a strange order and ended up removing the mirror modify, doing the UVs and then duplicating > scale x -1 and joining both side together. I’ve then gone and selected all UVs and “mirror UVs” but I get 250 duplicates found mirror may not be complete" and indeed a few of the UVs of the left side are left overlaying the equivalent UVs on the right side.

What causes this error? I can’t find anything unusual about the verticies or UVs that are creating the problem.

Also, is there a way to set UVs on the mirrored side to ‘save’ once you apply the mirror modify? I only seemed to have one side of the uvs with a mirror modify, and applying it didn’t generated a ‘mirrored’ uv set like I thought it would.

cheers.

I see no link to a blend file in your post