swarm-call: avoid duplicate active call on merge

If two users were starting a call at the same time, activeCalls
was filled with a duplicate active call. This was due because
the merge commit re-announce all commits where the linearized
parent potentially changed, causing a call to be added two times
in the vector.

Also add a fail-safe to avoid this in the future (will only print
a log in the back-end).

Change-Id: Ie419a419c13abcfb44e3c758fa40845f492504ae