In “Comment tu t’appelles ?” there is also two “tu”. The “t'” is also a “tu”.

“s’appeller” is a pronominal verb. So we need to put a pronoun.

“Comment vous vous appelez ?” or “Comment vous appelez-vous ?” is like “How do you call yourself ?” in english. And as you can see, in this question, you can find two “you”.

For pronominal or reflexive verbs in French (e.g. s’appeler, se laver, se demander, etc.), you start with the pronoun and follow with the reflexive pronoun, whether directly or inversely.

Reflexive pronouns are:

Je me

Tu te (not tu)

Il/Elle/On se

Nous nous

Vous vous

Ils/Elles se

The pronoun vous will always be used in conjunction with the reflexive pronoun (which also happens to be vous) in the case of reflexive verbs.

What may confuse you is the fact that the reflexive pronouns for Je, Tu, Il/Elle/On, and Ils/Elles drop the end vowel when followed by a verb that begins with a vowel.

So it will always be,

« Vous vous demandez / Tu te demandes peut-être pourquoi l’on utilise encore vous… »

« Je lui ai dit que vous vous appelez / tu t‘appelles Marius, mais… »

« Ma mère ne comprendra jamais combien il s‘attache / vous vous attachez à … »