Sounds like you are not noticing they are already present when adding,
I made the that error at first. What to look out for is the Blue Icon – if it dark – the FSID is already present somewhere in the tree. For Example if Carl Smith married both Sarah Jones and Megan Duclos – when you review Sarah and Megan you would only add ONCE from TreeShare – the the additional Spouse/Child/Parent connect should be done Add Select Existing Person
This is an easy mistake to watch out for if you are not careful. Family Search TreeShare does indicate its duplicate before you add but its not very obvious till you know what to look for.
The way I would go about this is SQL. From Within RM – I would probably do a Custom report then export to Excel if the list was large.
Off the top of my head I do not recall good way to work duplicate FSID within RM