Block recasting siphoning attacks while it's up.
 

Is it possible to create an addon that blocks recasting siphoning attacks while the previous cast is still up? This would be beneficial for PvP and PvE.

I think it should be possible yes.
There are addons already blocking necromancer skills out of combat:
https://www.esoui.com/downloads/info...alcasting.html

And this library (not used by the above addon though as it was created before that library) can be used to block skills:
https://www.esoui.com/downloads/info...llBlocker.html

This addon uses the lib:
https://www.esoui.com/downloads/info...nnerLight.html

Maybe ask the dev of this addon if he is willing to create another one for your skill.
Even better would be a general addon where you can blacklist skills and choose how long they will be blacklisted after usage (e.g. x seconds which could be the active time of the skill's buff).


Else I'd use an addon like
Action Duration Reminder
It helps a lot and should show the active skill time. Just does not block the usage. Maybe the author of this addon is willing to optionally inclue the library above to block skill usage as well.