Wow! You don't have ANY of the common pitfalls, and you're *still* getting a repeated audio track? I'm starting to think it's not an OBS problem, but you somehow have Windows' audio system configured weird, or another app is doing it, or something like that.
If you create a track in Audacity (no need to save, just play it from there and capture it normally) that has the shortest possible blip to see on OBS's meter (you might need to experiment with that), does OBS's meter jump twice?
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It might also be useful to record that in OBS and load it back into Audacity, and see if you do in fact have two copies of that blip. You'll probably need Audacity's FFMPEG plugin to do that, which is also free, but doesn't come preinstalled for licensing reasons. Once you have that, you can import a video file into Audacity, and it'll take the soundtrack.