As a workaround to the problem, I can suggest to following:

• Add a new field on the VOIP Phonecall module and call it "has_been_posted"

• As soon as you trigger the automated action, set has_been_posted to True

• Add "if not record.has_been_posted:" too your automated action

Please note: the above is a hacky workaround. Best solution is to find out why the automated action is triggering twice instead of once.