Just to add a bit more detail:
This is normal if you're capturing your desktop when OBS is an open application on your desktop showing a preview of your captured desktop in OBS. It's called the hall of mirrors effect.
It'll go away if you capture an application or application window instead of your desktop, or if you move the OBS window to a different display than the one you're capturing, or if you disable the preview, or tell the preview to open on a different display or in a separate window that you then move to a different display.