Beware. Serialization applies *forever* essentially. So if you set an
event, serialised to a user ID, then it will only be counted once *ever*
for that User ID. It doesn't cause them to be only counted 'once per
visit' as you've stated here. It would be better to use the 'Once per
visit' option for the event rather than Serialization, for this specific
use-case I think.