Echoing? Like two copies of the same source with different latency? I have a USB sound card that takes a finished mix from an external audio console, and feeds my headphones. OBS completes that signal path, so that the headphones tell me what OBS is actually getting, but the USB card also has a direct-monitoring switch. Turning that on while OBS is also active, creates two distinct copies of the same external feed, mixed together.
Or maybe it's a remote connection to someone else, that is feeding back to you, and you intentionally mix that in because you want to hear the other person...but they're also sending your audio back on the same channel?
Or maybe you're feeding the mic into some studio monitors, and that's getting back into the mic? That's the same type of feedback that a live audio engineer has to deal with. The solution there is simply to turn the volume down somewhere in the feedback path, re-aim things which uses their coverage patterns to effectively turn them down, or just cut that path entirely if you don't really need it. (unroute the mic to those speakers) Padding the walls can help too, if it's bouncing off of one and coming in from that direction, but the primary culprit is usually direct, not reflected.