I don't know if you solved this but had this problem myself today and found the answer buried on the vtiger discussion board yet still seems to be a lot of threads asking this with no response.
The problem comes, I think, if you migrated from 5.4 to 6, fresh installs should be fine. Basically it duplicates some of the import files when it installs. Backup and then remove the resources folder from modules > import in your vtiger install and it should all be sorted!