I've uploaded a gif to the stash:
https://stash.reaper.fm/37335/bug_overdub.gif
Seems that when recording over the edge of an item in MIDI-Overdub mode,
the item source is extended. On the first pass it is only extended by some
milliseconds, on the second pass it is extended fully.
Tested on Linux
Edit: New GIF (Using show overlapping media items in lanes)