@douwe It's likely that your link preview card is a 404 while the actual link works in the post - we had an issue with our docs site that presented the wrong absolute URL. It's fixed now, but Mastodon caches the wrong one forever, sadly.
On topic - deduplicating boosts is something that's on the roadmap Quite Soon and is not very hard to do, since we already dedupe posts internally. Arbitrary programmable post hooks are something best left for a plugin system, though!
We've released Takahē version 0.5! https://docs.jointakahe.org/en/latest/releases/0.5/
Image posting and hashtags are the headline improvements this time around, but there's plenty of smaller fixes. We're going to spend the next few weeks focusing on stability and community-required features, so people can start using it with more trust!
As a celebration of image posting (mostly) working, here is a takahē, the bird we named the project after!
They were once thought extinct, but are now part of a successful recovery programme that's slowly increasing their numbers (which you can donate to at https://nznaturefund.org/projects/takahe-recovery-programme/ if you like!)