Hi,
I've seen this problem too (I made an addon that scans the wealth statistics of players-
GoldScore) and I found your page while looking for a solution. I think GetFramesRegisteredForEvent might do the trick. As long as addons are friendly with each other. I'm thinking about something like this:
Let's say you have 2 addons that show some info when you target a player.
You target the player. Addon 1 gets the target changed event, and since it doesn't have any data to show in the tooltip, decides that it will inspect. It disables all other achievement ready callbacks.
Then Addon 2 gets the target changed event, and since it also doesn't have any data to show in the tooltip, decides to inspect. It disables all other achievement ready callbacks, including Addon 1's.
Then Addon 2 gets the achievement ready event and inspects. It reenables all other achievement ready handlers.
Now you will have to clear the target and target the player again; Addon 1 will behave just like before, while Addon 2 will have the data cached and not do anything, so Addon 1 will be able to do its job.
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It would be nice to find a way to avoid clearing/setting the target again, but I dont know if it's possible. Well, at least you avoid getting lua errors, so it's still an improvement.
I'll look into this when I have time, maybe next weekend. Should we stay in touch?