For future reference, change one, then test to see what happens.
The way a sync SHOULD work is that the system with the most recent changes overwrites the other system. So in theory changes could flow both ways in the same sync process. Now if the Google sync works that way, I can't say for sure.
However I did notice on the Google mail website (via computer or browser, not the gmail app) that there is a 'check for duplicates' option. So if the worst happens when you sync and you get all the duplicates back you might be able to more easily remove them using a computer and going online to you Google mail account.
Good luck, keep us posted.