Since the Import tool of K2 imports everything, Joomla Users and Joomla articles, and is designed to be used once, if you click it again, you will end up with duplicates.
Were the 700 users there when you did the original import?
Please backup and test first
I would probabaly take note of the mysql records i have now in K2 users and K2 items, K2 categories
and
do the import again after which i would
in K2 items and K2 categories Delete for id > noted number
but leave the K2 users
See friend request if you need help