Improve duplicate link checker to avoid the same event link in the same issue

[szabgab]

Apparently my pull-request was accepted and someone else also added the "Code Mavens" event in this issue: https://github.com/rust-lang/this-week-in-rust/blob/master/content/2024-12-04-this-week-in-rust.md Since then I sent a PR to remove the duplication from the next draft, but the duplicate link checker might be able to catch such events.