Periodic Event Actions are designed to trigger an action once per hour, daily, weekly or monthly. Under certain conditions it is possible for the action to trigger more than once.
If you edit and save an action that has already fired within the specified period, that action may trigger again. When you save the action, MassTransit reloads all the actions and re-initializes all of the time frames. Due to the re-initialization, MassTransit may detect that an action has not been triggered and it will trigger again.