If the data is being sent around the loop then this is a good error check, you just need to flush it from the received input data stream. If using VISA you can clear the input buffer using a VISA function (its in the Interface specific pallette) or filter the duplicated characters as needed by ignoring the character as its received, by comparing the character with what was sent (this would be the error check).
I guess that it's a master slave arangement so the remote devices are responding to commands from a controller, the P.C.)