Is there a reason I have both fallouts in a "normal" and a classic version in my library?
It seems both are exactly the same in terms of installation size but the classic versions offer additional goodies.

Edit: I did find an explanation in a different thread.
But one question still remains: if both are almost the same why aren't both merged into 1 item?

Because two different companies (legal entities) own the versions. To merge them would require all kinds of pointless legal agreements.

You shouldn't have both versions, unless you bought both. So why would you do that?

Indeed I did buy both as I've just checked my order history.
I bough the "normal" version without checking my library first because if you have the classic versions the "normal" version aren't being flagged as already owned.

Similiar to a purchase of an expansion pass like for Pillars of Eternity and both expansions are still being displayed as not owned altough you own them through the expansion pass.