Some reading about this topic:
http://www.graphisoft.com/support/archicad/archiguide/guid.html
To solve this, do something which will cause the GUID (see above article) of either object to change:
1. Select one of the files and open it for editing
2. Do not modify it, just choose Save as... from the File menu and save it under its own name. so it will overwrite itself.
This will result in the GUID being altered by the program.
3. Go to the Library Manager and delete the reference to this file from 'Other Objects' the list of loaded libraries
4. Reload the library
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