I just realized I have many many duplications in my file - both the online tree and the Family Tree Maker version on my Mac. Argh!!
It should be possible to create a utility that looks for duplication and gives you the opportunity to keep or delete it - while retaining the links to the people you want to have the 'kept' version.
In the meantime, I'm going to stop trying to remove duplicates from the online tree AND my FTM tree. PLUS I'm going to reduce how often I sync, since that has to have something to do with it.