Bukkit

What steps will reproduce the problem?
1. Summon donkey/mule in overworld
2. place item in chest inventory
3. dismiss it
4. summon animal again
5. walk through nether portal
6. walk back
8. grab items from animal's inventory
9. summon animal yet again
10. viola, duped animal with duped inventory = sad admin

What is the expected output? What do you see instead?
The animal should be dismissed when player enters the portal, but it doesn't.

What version of the product are you using?
happens in 1.1.11, 1.1.12, and 1.1.13

Do you have an error log of what happened?

Please provide any additional information below.