Either service could check if someone is already on a list before adding/notifying someone but it would add additional costs (either to Bluesky API usage which has limits or local storage of list memberships that then might get out of sync with what's actually on Bluesky).
It's also important to note that there is no harassment without the duplicate notifications from the service. From the normal Bluesky interface without that service, you can't even see what lists you're on and especially not that you've been added multiple times.
Thank you for all. I appreciate it.