When loading their worklist, from Workspace or a worklist control, a user may encounter the following issue: "Item has already been added. Key in dictionary: 'Action_Name' Key being added: 'Action_Name'." This may prevent a user from accessing their tasks.
Please contact Nintex Support to get assistance with resolving this issue. A profiler trace will need to be run to extract the worklist query for the user and then some analysis has to be done to determine which process instances need to be redirected back to the step where they are at using Go To Activity.