I haven’t been able to find a solution to prevent alarms being resent if the gateway restarts.
By default, “Notify Initial Events” is false for alarms:
https://docs.inductiveautomation.com/display/DOC81/Gateway+General+Alarm+Properties
Has someone changed it to true on your system?
No, it is still set to False 
With notify initial events set to false, then all the alarm events persisted when the gateway stops should “re-trigger” when the gateway starts back up again, but not send out notifications. If that’s not what’s happening (as it sounds like) you should contact Support so they can figure out what’s going on.
Did you come to any solution for this? Or did support help you investigate? I have a similar issue with edge gateways restarting and upon restart resend their alarms even when the “notify initial events” setting is set to false on all the gateways.