Insert entry in an empty database leads to KeyCollisionException

[koppor]

JabRef 3.3dev--snapshot--2016-03-18--master--57963f7 on Windows 10

When I delete an entry at a database and refetch the entry via the DOI to BibTeX converter, the entry is marked as duplicate in the fetch dialog.

Steps to repoduce:

1. Create new database
2. Search for 10.1007/978-3-642-36926-1_34 in the DOI to BibTeX fetcher.
3. In the untitled dialog for adding entries, the arcticle appears
4. Click "OK"
5. Wrong behavior: The checkmark in "Keep" disappears
6. Add the checkmark at "Keep"
7. Click "OK" again
8. Wrong beheavior: nothing shown at the UI
9. Click "Cancel"
10. Open Error console

Exception in thread "AWT-EventQueue-0" net.sf.jabref.model.database.KeyCollisionException: ID is already in use, please choose another
    at net.sf.jabref.model.database.BibDatabase.insertEntry(BibDatabase.java:163)
    at net.sf.jabref.gui.ImportInspectionDialog.generateKeys(ImportInspectionDialog.java:489)