All your contacts will be rescanned on a regular basis, meaning you'll never have to worry about duplicated contacts again.
Contact Merger works offline and ad free. It was written / published because I needed a tool to merge more than 500 duplicates.
The source code is available at https://github.com/rtreffer/ContactMerger
Issues can be reported at https://github.com/rtreffer/ContactMerger/issues
You can now cancel scan
Decreased scan interval
Fixed one bug that would cause repeated scans
Fix one crash (please report crashes)