Yes, I thought of that problem. However, I don't see why that type of loop would not occur when posting with the web interface too.

I would think the only way to avoid that problem, if multiple crosspost-enabled journal sites exist, is to have the server be able to recognize when it receives a crosspost request for an article it has just sent out as a crosspost itself, and stop the loop. Neither a standalone client nor the web client has the ability to detect and stop a crosspost loop, but the server does.