XUI: Sharing resource twice (with another user) fails

Steps to reproduce:

1) Register an UMA resource as user X
2) Go to OpenAM dashboard and manage shares
3) Share resource with user Y
4) Share resource with user Z

An error is displayed and an exception is in the logs.

Note, that if the user shares with user Y and user Z at the same time (in the same dialog) it appears to work.