When you try to add an already existing tag, Astral takes you back to sign in

[Andrea Panizza]

Hi,

first of all, thanks a lot for the amazing tool! It's great to find my way across the many repositories I starred. So, here is the issue description and procedure to reproduce it:

1. sign in to Astral
2. try adding a tag which already exists in your tag list
3. The app takes you back to the sign in page

I was taken back to login a few times before I realized I already added that tag in the past, so that may be confusing, expecially for new users.

System
OS: macOS 10.15.7 (19H1323)
Browser: Chrome, version 92.0.4515.159 (Official Build) (x86_64)

Proposal
When the user tries to add an already existing tag, capture the error & show a (popup?) message which says "tag {s} already existing", rather than just crashing and exiting the web app.