Usually, the problem occured due to a bug in the tagged value handling that is fixed in the current release. The duplicate value is pointing to two elements carrying the same information. Simply removing one of them would solve the problem here.
Having two different elements being pointed to is a more tricky situation. You can give one of them a new, unique id (a number greater than the biggest one in the file would be a good idea here). But you also have to make sure that all the references are now pointing to the correct element. In case the elements are completely different (e.g. a class and an action state), sorting out the right one is quite easy. Otherwise, you can make a good guess from your knowledge about the model itself.
After all references have been adjusted, zipping the bunch of files (.argo, .xmi, .pgml) into one file with the extension .zargo allows you for easy reloading it into Poseidon. Please note that due to bug in the xml readers loading a correct project file after trying to load a corrupted one might not work as well. A restart of Poseidon will solve this problem.
If you cannot figure out how to repair the file, send an email to
info@gentleware.com and attach the corrupted project file. We will then try to repair the project if possible.
Thorsten