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!

I would like to see this question answered by one of the site teachers as the usage presented in this lesson does not seem consistent with that presented in sites such the link offered by Chris, nor with texts such as Grammaire progressive du Français.

Hi everyone,

I don't think 'dont ' and 'duquel' are always interchangeable.

In the examples given in the lesson , the verbs all use de :

avoir envie de , avoir peur de, parler de, avoir besoin de ...

In thoses cases you can use any of the possibilities suggested .

Hope this helps!