I had the same problem.
I’ve try many things before, so I’m not 100% sure it’s this final solution that worked. But I think so.
The trick is using the compound clip function. I didn’t knew that you can copy a compound clip from a event to another. You can. So:
1) open your unduplicatable project, select all clip and compound it. It will appear in your event (let’s say event A)
2) In the event browser, create a new event (called B).
3) take the compound clip from event A, drag it in event B : It’s suppose to copy affiliate clips in the new event (B).
4) Associated with event B, create a new project, put your compound clip from event B in it, uncompound it if you like.
5) go in the project library, right-clic on your new project and clic “consolidate project media”
Bouya. It worked for me. Maybe by copying compound clip fcpx didn’t care for clips names and rename it for me, it’s hard to tell for this project: to many clips came from a derush from a media manager made on fcp7, so I have many clip with -1, -2 , -3 extension.
I don’t have the time right know to double check.
Sorry for my english.