I know this is an old thread, but I am experiencing this issue when I try to update my contacts through the API. It will update fine, but if I include a changed e-mail address it inserts a new record. To do the udpate, I GET the XML for the user using their UserId. THen I modify the XML with the updated values. Then I PUT the user informaiton. If I don't include the e-mail address the user gets updated. If I include the new e-mail address a new user gets added. I would prefer not to remove the user and then re-add them.
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