This is duplicate issue....check this table W_WRKFC_EVENT_TYPE_DS you can find duplicates...delete duplicates and restart your execution plane, it will work.
For this follow below process:
* Goto dac design tab>indices--- search for failure index in that list.
* You can find unique columns , using these unique columns make a query to find out duplicates.
* Delete duplicates and restart your execution plane.
Let me know if you facing anymore issues.
i don't think root cause should be from mapplet...first confirm that your source system don't have any duplicates.
If there is no duplicates in source system means root cause should be mapplet. But i am sure that there is no issues with OOTB's, may be your source system have duplicates .