Are notifications allowed to have side-effects?

[michielbdejong]

A little philosophical question I ran into while writing the test suite:

When Alice shares a resource with Bob, she cannot share it again. If she tries to share it a second time, her GUI will warn her 'Bob already has access'. But if Bob sent back a notification like SHARE_DECLINED or SHARE_REMOVED the that will in practice usually have the side-effect that Alice can share the resource with Bob again.

This suggests that either sharing with someone should be idempotent, or the notifications and their side-effects should be standardised.