"Dont" has a broad range of applications compared to "duquel" and its brethren. It is used as a relative pronoun to signify possession as well as being part of a group:

C'est l'auteur don't j'ai lu tous ses livres. -- This is the author whose books I've read all. Here "dont" establishes the fact that the books "belong" to the author, i.e., they are written by him.

J'ai 5 chats dont 2 chattes. -- I have five cats, 2 of which are female. You use "dont" to show that the 2 female cats are part of the 5 cats.

In all those functions, you can only use "dont" and never "duquel". Where the confusion starts is usually when it comes to verbal vs. prespositional phrases. Here is the rule up front: Use "dont" in verbal phrases and "duquel" in prepositional phrases. Let's have a look at what that means.

Verbal phrases always include a verb + de: parler de, avoir besoin de, etc. In these cases, use "dont" like this:

Je parle du garçon --> Le garçon dont tu parles.... -- The boy about whom you're talking...
Tu as besoin d'argent --> L'argent dont tu as besoin... -- The money that you need...

Prepositional phrases, as the name suggests, include a preposition along with a verb:

être assis à côté de qqn: L'homme à côte duquel tu es assis... -- The man next to whom you are sitting...
être à droite de: L'appartement à droite duquel mon appartement est situé... -- The apartment to the right of which my apartment is located...

Here is a link that explains it in a bit more detail: https://lovelearninglanguages.com/2021/05/13/french-relative-pronouns-dont-vs-duquel/

Hi Carol,

I also find this aspect difficult to get right every time.

Try here:    https://www.wordreference.com/fren

Enter "dont"   and then "duquel"    the various aspects of each usage may help you.

The lesson is also very good of course, but it can be helpful to reference another source to reinforce comprehensive understanding. 

Hope this helps,

Jim

This is in response to Chris’s Correct answer starting "Dont has a broad range of applications". He goes on to say:

C'est l'auteur don't j'ai lu tous ses livres. -- This is the author whose books I've read all. 

.. and I’m just wondering why it’s "ses livres" and not "les livres" here?