Él es quien probablemente tendrá éxito.

This one confused me slightly. Normally if I were saying this with a specific person, I would say something like “Ricardo es él quien probablemente tendrá éxito.”

Is the second “él” omitted here so as not to be redundant? And is this a legit thing to do in general? Is the “él” mandatory if you have a different subject of the sentence?

You can say either “el que” or “quien”, but you can’t mix them. And “él quien” would only work if you said “Es él quien tendrá éxito”.

Disclaimer: I’m not a native speaker.