Most commonly this happens when someone is watching their stream from the system they're streaming from.
The video playback is picked up by the Desktop audio capture in OBS, sent out again, and boom, looping.
Second-most common is using Audio Monitoring in OBS, with the Monitoring Device set to the same thing as their Desktop device.
If you aren't watching your stream from the same machine you're streaming from, post a logfile and we can take a look and see if something is happening on the back-end.