Hi Jochen,
It's an interesting situation and to be honest, I don't know for sure how search engines will deal with that "link juice". This will come down to a question of whether search engines see robots.txt or htaccess first. If it looks at robots first (which is my suspicion), it can't see that page to pass the strength.
I suppose to test this, you could submit the redirected page to index via Search Console and see if it shows you the redirect or says it's blocked.
Interesting question aside, there's no real need to block access to a 301'd page 
Also, apologies if I'm just highlighting the obvious here but it would be far better to clean up the site structure and remove that duplication rather than just masking it with robots; the user experience is at least as important as the algorithms!
Along the same lines, cleaning up those pages is going to help your crawl budget immensely.