Find both copies
Elgato has replaced Wave Link 2 with Wave Link 3, which is also free. These steps follow Elgato's Wave Link 2 articles. In Wave Link 3 the test is the same (mute one copy and listen), but the menu names may differ.
While the sound plays, mute its direct OBS source. If one copy disappears but the sound remains in the Wave Link mix, that sound has two capture paths. Choose the intended one.
A capture card can feed Wave Link while also appearing as its own OBS audio source. Disable the duplicate direct path if you use the finished stream mix. If you want separate OBS sources, exclude that sound from the mix OBS captures.
Elgato's recommended fix is to mute the capture card in Wave Link's Stream Mix and keep its OBS source unmuted. The other option is to mute it in OBS and keep it in Wave Link.
Check monitoring as a separate loop
Inspect OBS Advanced Audio Properties and its monitoring device. Monitoring Enabled (Monitor and Output in older OBS versions) can create another return if that device is captured again. Turn monitoring off for a trial; this does not replace checking the stream mix.
Use headphones and test the audience recording separately from Wave Link’s monitor mix. Loopback and mix-minus explains why the monitor and stream destinations can differ.
Replay each source alone
Record speech, game and capture-card sound separately. Listen for delayed copies or phasing. Then record them together using the routing plan you intend to keep.