@dc42 Thank you, fully understood and will solve the problem.

You have some other problems. Twice there is the sequence:

es after '''S''', expected 3
Done!
Response while disconnected:

The first line is probably part of an error message from the Duet, but Repetier has chopped off the first part. I've no idea about "Done!" but I think that's from Repetier, not the Duet. After that, Repetier seems to think it shouldn't be connected and is surprised to see a response.