Is your feature request related to a problem? Please describe.

At present, if you use the available add-ons category, add-ons already installed are not shown in the list. I am assuming for this discussion, that the user is using the correct category such as all or stable when looking for the add-on.

Describe the solution you'd like

All add-ons in the category, such as all stable add-ons, should be shown in the list when in available add-ons. Available doesn't mean only uninstalled. It means all add-ons offered in the store in the category..

Describe alternatives you've considered

None.

Additional context

The current method is going to cause a lot of confusion. I check, from time to time, to see if there is an add-on for something someone asks about on a users list. If I already have an add-on installed, such as Clipspeak and I look in available add-ons, I won't see it. I may conclude that it is no longer offered in the store.
The correct way to do this is to show all add-ons in a category, then if someone mistakenly tries to install one that is already installed. bring up an error message that the add-on is already installed. Or the add-on could show that it is installed next to the name such as, Clipspeak, installed.