When having (an insane) diagram like:
Alice <-> Alice : text
Alice <->o Alice : text
Alice o<->o Alice : text
Bob <-> Alice
Bob <->o Alice
Bob o<->o Alice
the bottom 3 look all OK
the top 3 are all having just one arrow head (the circle seems to be OK).
Even though it is a bit an insane diagram I think that the both arrow heads should be present.
(plantuml version 1.2021.7beta1)