Also, do 2 (or more) of the xrefs bring a nested (attached) xref into the same DWG? E.g. Main.DWG has Second.DWG + Third.DWG xreffed in (either attached or overlaid - doesn't matter here)
Then both Second & Third has a Common.DWG attached into them.
When binding Second & Third into Main, you'll find that bind will error out because in effect bind converts an xref into a normal block. So while attempting to convert the Common.DWG xref into the Common block - there's a name conflict.