Avoid duplicate error notifications on screen

[fregante]

As seen in https://pixiebrix.slack.com/archives/C0436P48QHY/p1705451796174779, the user might occasionally be presented with walls of notifications and errors.

I think we can relatively easily avoid this:

1. Remove an identical notification from screen before creating a new one
2. Add a counter, on the icon or in the text (easier)

   An error occurred adding 1 context menu item (s) (x6)

[fregante]

I'll check if the code allows deduplicating this specific notification first, e.g. by coalescing the errors