It’s a bummer but there seems to be no way of disabling repeat_interval for alertmanager currently - quite annoying oversight if you ask me, in terms of maintaining the same alert behaviour after migration.
I think the only reasonable workaround for now is just to set some ridiculously high humber like 999 weeks - the counter resets if the alert resolves IIRC so it would only repeat if alert is actually firing for that long.