To reproduce this issue:

1. Open the attached project
2. Open 'Test event' scene
3. It has a script which creates Unity Event and adds 2 listeners
4. Play the scene
5. Press 'T' which invokes event
6. The first listener receives the event ( "Test" is outputed in the console)
7. Then first listener removes itself from the listeners
8. The second listener does not receive the event
6. Press 'T' again
7. Now the second listener receives it ( "Hello" is outputed)

If you do not remove listener after receiving the event, everything works fine and both listener receives the event