Bug 345823

There is strange notifier behavior when we repeat several times 'apt update' command. If there are some packages that are going to be updated, notifier renders several times message about available updates.

Also:
the systray icon that informs us about available updates is gone just after we execute 'apt update' - this looks like some status reset here, but that is fine. When packages are updated we should have only one notification about available updates, not several as mentioned before.

For details please see attached link to video.

Reproducible: Always

Steps to Reproduce:
1. Make sure there are some packages to be updated
2. Run command: apt update
3. Notifier will render the same message several times, each time we run the command

Actual Results:  
Notifier renders same message (about available updates) several times.

Expected Results:  
Notifier should render only one notification.