Diagnosis
The request has either been updated to the data target already or is atleast available in the PSA already.
You cannot post this request again.

Procedure
This data package of the request was probably processed with errors inthe first update. Therefore, the data package remained in the TRFCqueue of the source system and is ready to be sent again from there.
Check whether this is the case and delete this data package from theTRFC queue of the source system if necessary.