Are these really equivalent?

I keep seeing material that talks about the importance of distinguishing between them:

Dont for a phrasal verb : Le livre dont je t'ai parlé

Duquel/etc. for prepositions : Le livre au-dessus duquel j'ai renversé mon café

The distinction is a pretty complicated matter and depends also on the register you are using. Here is a page which explains it and also offers a little test in the end to check your understanding: https://www.francaisfacile.com/exercices/exercice-francais-2/exercice-francais-105504.php

-- Chris (not a native speaker)

Thank you, Chris!  I did look at the Français Facile page the other day, which is why I was surprised to see the two possibiities described as equivalent.  

I don't see them as interchangeably equivalent. They serve a similar purpose but in some instances you need one where you can't substitute the other. I believe the site to which I postet the link in my previous post explains it pretty well.

-- Chris (not a native speaker).

Thanks!