Jim,
To answer your questions...
1) It will not duplicate accounts as long as you run the product matcher prior to running the synch. The Account Matcher finds exact matches of Account/Customer names and links them between the two applications. Duplicates will be created if one of two things happen. First, if you have 'ABC Company' in Dynamics and 'ABC Inc' in QuickBooks the software will not be able to link them during the Account Matching and will create a duplicate in Dynamics. A duplicate can also be created if you have 'ABC Company' in both QuickBooks and Dynamics but neglect to run the Account Matcher to 'match' the names together prior to the synch.
2) DataSynch will match exact Dynamics Accounts to exact QuickBooks customers as stated in the answer #1 above
3) If there are two accounts with the same name, that's an issue. You'll need to change one of the names.
4) No, the fields we synch out of the box are the only fields that will be synched. We do not synch custom fields.
Hope this answers your questions...
Chad