Prevent multiple instances of the same feed from being added

[SamTV12345]

Discussed in #426

^{Originally posted by shooftie November 15, 2023}
I was having trouble uploading my OPML and have managed to add 5 instances of many of the feeds.

It would be helpful to prevent the same feed from being added multiple times, additionally, it would be even more helpful to throw up a notification that this was the case.

[shooftie]

OK, so I did some testing to see whether I could find a pattern.

I removed all but one of the feeds from my OPML to see whether I could reproduce it. I could not. It would appear that the issue might be related to:

1. the fact that I am upload so may at once
2. the fact that I was using an old image (i can't remember whether I had this issue before or after you fixed it in Fixed docker-compose with correct tags. #419)

What I did manage to do, however was this: (apologies for the quality, there is a 10MB limit on uploads)

Screen.Recording.2023-11-15.at.20.59.15.mov

Remember, this is an OPML with just a single feed in it. export 2.opml.zip

[shooftie]

I cannot work out any rhyme or reason as to which feeds duplicate and which do not...

• 99% Invisible: last release 1 week go – duplicates
• A Brief History of Mathematics: last release 13 years ago – duplicates
• Inside Science: last release 6 days ago – does not duplicate
• Motormouth Recordz: last release 4 years ago – does not duplicate

[Brausmith]

I've got the same duplicate issue. But I suspect in my case this might be related to my mobile device gPodder sync.