My Quicken did not duplicate it, just broke most of the transfer links for May, and now I can't reconnect them. If I try to fix them it says, It says "this transaction has one or more invalid transfers." If I click edit to fix, it won't let me; the category is gone. The only way to fix that I have found is I have to manually add in all the transactions that the transfer broke, relink them, and then delete the "broken" transactions.
I am on Mac 9.0.3 Build 900.60651.100