System.Data.DuplicateNameException: A column named 'APPROVAL4_dtVacationEndDate' already belongs to this DataTable

Hi everybody.

I hope all doing well.
last time I tried to apply an action to an activityInstance using wfgen web service as below:

POST /wfgen/ws/ProcessesRuntime.asmx/CompleteWebFormActivityInstance?impersonateusername=username HTTP/1.1


Content-Type: application/x-www-form-urlencoded; charset=utf-8

Content-Length: 586

processInstanceId=24857&activityInstanceId=7&workflowContext=APPROVAL4_rbDecision4TEXT INOUTموافقةAPPROVAL4_dtVacationEndDateDATETIMEINOUT2021-01-25T00:00:00+02:00APPROVAL4_dtVacationStartDateDATETIMEINOUT2021-01-01T00:00:00+02:00

but I faced the below exception :

HTTP/1.1 500 Internal Server Error

System.Data.DuplicateNameException: A column named APPROVAL4_dtVacationEndDate already belongs to this DataTable.
I searched a lot in the forum and the web but I couldn’t find any answer.
kindly may you advice me to fix this error.

thanks in advance.