I have just installed eM Client as a possible replacement for Thunderbird.
My contacts are all held in Google Mail so that I can synchronize them with my Android devices. There are about 150 contacts. However, when I set up eM Client with my Google account the address book has grown to over 6,000.
Only two contacts appear to have been duplicated. One has about a dozen occurrences and another has about 6,000. The duplicates are now also present in Thunderbird so they must have been written back to the Google database but, oddly, the duplicates are not visible using Google Mail through its own web site.
I now have to find a safe way to clean up the mess. If all these bogus contacts are copied onto my Android phone it will probably cause major problems.