Hola Marcos

Sorry, your second message went through the cracks...

When you are using a direct object pronoun you can't repeat the object that way:

¡Cuántas veces (la) he visto ella! --> incorrect

You can only say here:

¡Cuántas veces la he visto!

So, you can only use the pronoun once and cannot then use the "subject pronoun" (ella)

The same here:

¡Cuánto tiempo (la) he tenido ella! --> incorrect

You can only say here:

¡Cuánto tiempo la he tenido!

The only time you can repeat the object is when we start by introducing the sentence with the object itself, this way:

It does sound ungrammatical but we do use it this way. It is like saying:

Russel Crowe's film, I've seen it many times.

This guitar, I've had it for a long time.

I hope this clarified it.

Saludos

Inma

Hola Marcos

A tricky one...

For your first sentence "(Los) saludé a todos." (the pronoun would be los, not les) we do need the direct object pronoun "los", simply because it is a requirement when we use "todo/todos" as a pronoun, and this is what R.A.E says (go straight to point 16.14 n)

Other similar sentences are for example:

Lo he visto todo.

I saw it all.

Leí todas las páginas -> Las leí todas.

Ahí están tus amigos, los conozco a todos.

There are your friends, I know them all.

For your other two sentences:

- (La) busco a alguien.

- No (lo) ha visto a nadie.

The R.A.E (as far as I know) doesn't have a specific chapter for this, but I can tell you as a native that using the direct object pronouns (lo, la...) is wrong in these cases. I imagine it is to do with "a alguien", "a nadie" being indefinite pronouns, so we can't use another pronoun either masculine or feminine if we don't really know the gender in "a alguien" or "a nadie"; so the correct sentences would be:

Busco a alguien.

No ha visto a nadie.

Saludos

Inma