I didn't mention "Merge your Duplicate contacts" because I found it actually caused more duplicates when used in conjunction with Google, MfE and Nuevasync, but it is a very useful tool on the N900.

I use "Merge..." to double-check for duplicates but do all my editing & merging on Google's webapp and it seems to work better - I am currently not getting and duplicate contacts and only the occasional field duplicated within any of my 500-odd contacts syncing between my N900 and N8.