In any case, Jeremy describes how to generate a new GUID over here, but I couldn't find GUIDgen.exe easily on my XP 64-bit PC, so I downloaded GUIDgen from:
- Generated new GUID, copy to clipboard
- Open offending (ie. most recent) .addin file that caused Revit to show the error on startup
- Paste the newly generated GUID into the ClientID field
- Save the .addin manifest and restart Revit
This resolved the GUID conflict.