The following error appears when deploying a workflow: 
Missing reference at Activity ‘Task’. Error. A item with this name already exist on this container



When you add three or more actions on a workflow task step, you might receive the missing reference error which will prevent you from deploying the workflow.

Troubleshooting Steps

This issue has been fixed in K2 Five 5.1 FP 4:

If applying the fix pack does not fix the issue, you can use the following workaround:
Rename all the Task Results in 'Results/Voting' tab in the Task step.
- Example: Approve > Approved , Cancel> Cancelled