Actualy 'safe' building of the base has in view layers (as part of `BKE_main_collection_sync_remap`) would only happen when there was already an existing one, otherwise it was skipped, and rebuilt later (without the support for doublons) in collection sync code. Very odd that that error was never spotted before, issue in code has been there for a long time already. Probably only happens in rare cases (specific conjuction of factors during remapping of old ID into itelf new id)? Reported by @hjalti from Blender studio. Reproducing case: `heist/pro/shots/050_alarm/050_0160/050_0160.anim.blend`, r1407