The trust holder for the company's defined benefit plan issued duplicate checks for the plan's consultant services. The consultant cashed both checks. The consultant is now issuing a check to the trust holder in the amount of the duplicate payment. Does this scenario constitute a prohibited transaction? Are there any reporting obligations required on the part of the plan? Thanks.

You should let their attorney make this call.

Effen is being cautious. No disrespect, but I suggest common sense.
You should not have to spend $ on legal advice to figure out a simple solution. This is a simple "oops", and a simple solution is appropriate. A reimbursement check sounds simple, and advisable.