Name the physical input
Choose the microphone that should supply speech and identify its actual device name. A source called microphone may point to a different input or to a system default that changed. Keep the listening destination quiet enough that speaker feedback does not confuse the first comparison.
Find every capture route for that device
OBS’s Audio Sources guide warns about selecting the same device both as a scene capture source and under Settings > Audio. Inspect those routes together. If the presentation needs scene-specific microphone activation, decide which route should own it and preserve a record before removing an unintended duplicate.
Do not start with a compressor, gate or noise filter. Those process the captured signal and do not establish whether the input was acquired twice. Make the ownership correction first, then evaluate the ordinary sound.
Confirm through a saved sentence
Record a short spoken sentence through the intended route and listen to the saved output. Check another scene where the microphone should be active and one where it should be absent. This verifies the chosen capture scope as well as the immediate sound.
If doubling remains, keep that narrower result and investigate the remaining route. The documented duplicate-capture warning does not prove the cause of every echo. No microphone or recording was tested for this article.
Questions answered
Can duplicate capture exist even with one physical microphone?
Yes. The same device can be selected through more than one software route.
Should I use a noise gate to remove the duplicate?
First establish one intended capture route. Filtering does not identify duplicate acquisition.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.