Hi friends,
what happens if I purchase a bundle of which I already own a title, e.g., the current Metro bundle of which I own Metro 2033 Redux? Will I get reimbursed for this title?

You should receive an email containing a gift-code for the item(s) you already own.

There are exceptions like the recently published
RPG
and
Strategy bundles.

If you own any of the games included in any of those, you will not receive a key and can't gift any of the games. You will usually find a warning at the bottom of a page when this is the case.

Thank you!

I know in the past they have removed the discounted price of the games I owned from a bundle and only charged me the price of the games I hadn't already bought. Oddly, the price didn't show this change until I actually went to my payment processor.

But gog has a way of doing things differently over time, so don't count on what I've said.

Does anyone know how it works for the new Mafia bundle if you already own Mafia III?

Sorry but you are wrong, none of GOG's pre-built bundles provide extra keys in case you already own one or more of the games contained in the bundle. When you make the purchase there is even a box that lists the games you already own and warns that you won't receive extra keys. In your specific case, the old Mafia III is a separate game from the new Mafia III: Definitive Edition, so you'll have two Mafia III in your library.

Not to offend, but this seems a -lost in translation- case...
You may give another read to Mori_Yukis post (I added the bold format)

And why am I here interfering in a not my business?
It happens I am here to reply thanking Mori_Yuki, which next to your post... collides.

So when i bought the Myst anniversary bundle and i had Uru: ages beyond Myst bought previously and didn't receive a gift code for the game i already owned.

if you have mafia 3 its a free upgrade to definitive, get 1 and 2 separately, there is a good chance it'll be cheaper than the bundle regardless.

The Mafia III in my library is already the Definitive Edition. I guess I might get the bonus copy, I wish they would just discount the bundle by like 7 bucks though instead.