I ran into duplication as I had both (default) Desktop Audio enabled, as well as the specific audio source (mic)... ie a duplicate/echo
There are good source materials out there on the sophisticated Audio mixing and monitoring possible in OBS... it is easy for someone who hasn't RTFM to cause themselves trouble (like I did) [someone posted some great links to guides on these topics recently, but I failed to save them and now I can't find them... maybe someone will be nice enough to post them]
For some use case, my approach won't work, but I simply disabled all Global Default audio sources, and added Audio Input sources per scene only as specifically required... that ended my issues
Oh, and Win10's new Audio handling control panel /system apps are a lot more involved, with privacy settings and more... Then add 3rd party motherboard audio s/w (like Realtek)... ugh... I've tripped over Audio more than anything else in my OBS journey
As for Monitoring on - typically not done for you own microphone. But enabling audio monitoring for pre-recorded content (ex when I play a video) can be helpful
