Sounds like a setting changed...or you're using the Default audio device somewhere and Windows changed what that means. It does that without warning or indication, except that something suddenly doesn't work anymore. So don't use Default. Always choose a specific device *in OBS*, and then it won't change on you. Or if you're not using something, Disable it or Remove it. Don't leave anything on Default, at all.
Anyway, the most common source of an echo is Desktop Capturing or Audio Output Capturing the same device that the Monitor is sent to. The capture catches EVERYTHING that that device does, including the Monitor, and so whatever you send to the Monitor also comes back in that source, usually with enough delay to be perceived as a separate echo because Windows' audio is that slow.
