Why do MIDI items loop in overdub mode and how can I stop it?

I want to use "MIDI overdub in existing items" but when recording, the items loop automatically when this mode is selected, which I definitely don't want. What am I missing? I've never noticed this behavior before. Thx.

I should clarify that by loop, I mean the previously recorded part repeats and the it takes on the familiar loop niche as the track scrolls--really want to turn this off so that I can just record new MIDI in a track without overwriting what's already there!

Can you post a screenshot? Maybe that's easier to grasp, not sure what you're referring to. Did you reset your REAPER.INI maybe and have "Loop source" enabled for new items in 'Prefs->Project->Defaults' now?