If the loop is more than about 1 second or so, then it's almost certainly because you're watching the stream with the audio on, and that's getting picked up again. Mute the stream return, and you should be good.
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If it's less than 1 second, then it's probably a local loopback. Most of the time, that's caused by Desktop Capturing or Audio Output Capturing the same device that you send the Monitor to. Those captures catch EVERYTHING that the selected device does, with a pedantic definition of "everything". So yes, OBS's own Monitor is included.
The solution there, is to use different devices for the Monitor and the Capture, and design your rig to still work with that requirement, or, don't Monitor anything if you can afford that.
Windows also has a setting to "listen to this device", or something like that, which might also be part of the problem. But check OBS first, as it's easier to get to.