The lightmap should still build even with overlapping UVs, the mapcheck will just complain about it and you’ll get errors in the overlapping regions.
Most likely your geometry overlaps within islands or is mirrored, could also just be a really small triangle somewhere that is difficult to see. Basically issues that can’t be fixed by regenerating the lightmap UVs in the editor.
If you build with error coloring it should highlight where they are;
