Duplicate Borderlands GOTY Activation

This game has two versions
Old (not available for purchase, but there are old keys) and new
Everything is fine

I got this as well with Warhammer: Vermintide 2 - have the free copy that was given away on Steam a year ago, and then a retail one - was instead expecting the retail one to say it was a duplicate. No change in Steam Library, no DLC added etc. Steam Support doesn't seem to have any category to remedy this, and in the licences section, it can't be removed, so key's basically used up for nothing unfortunately...

Not a bug, some publishers love to bundle ""non-existent" new content to their old games. That way they technically activate different content than the old keys activated, and people can activate already owned content's keys without gaining anything. If you check back older Paradox bundles, there is usually such a warning in the OP, that you can activate the keys even if you own the content already. Likely a similar thing is going on, usually you can activate a game's bought key on top of the already owned free license, not everyone is like Paradox.

https://www.steamgifts.com/discussion/Rxx1q/humble-bundle-storied-strategy-and-role-playing-bundle-paradox-turn-based-bundle

"Paradox packages always have hidden apps that enable you to activate a key on top of a store purchase, for the exact same content. Make sure to not waste your keys by redeeming them this way."

Sorry to be off topic, but since you mention a possible bug, I'd like to mention something that bugged me.
I noticed lately, games that are on sale, without mentioning until what date.
Games like: Obduction, Opus Magnum, Machinarium,...
If anyone knows anything, please let me know.