Sometimes a GWCheck/Mailbox maintenance run of Contents Analyze/fix on the GroupWise side, PostOffice or individual mailbox, will clear this problem. Which is why you should have one running for each PostOffice about once a week.(structural daily)

It will show in the maintenance log as either  (see them near the bottom)

   79 Duplicate folder names at same level......
  102 Contact folders out of sync...................

You can find the individual user(s) by searching for "Problem 79" or "Problem 102"

Any they don't clear from those runs, then they need direct inspection of the user's address books to remove duplicates and make sure any remaining books have at least one entry in each.