There are a couple of places you can check to see if this is already in a slip or posting batch, if it isn't in the below places then delete the draft Slip and recreate as it could be a change in the WIP allocation due to reversals or transfers of the WIP on the Desktop:
Go into Slip Posting from the Main Menu and select the TO POST tab.
Check for the matter and select DELETE if there.
Also check the UN'AUTH, REJECTED, ANTICIP'D, UNALLOC'D, DRAFTS tabs.
Check in General Postings that there are no saved batches.
If there are any that relate to this matter then select DELETE.