Sure, "Tu" might be superfluous, but since it is there, it is there for emphasis. It could be part of a quarrel: "- YOU joined US. WE did not drag you with us ...." -- except that Italian passato remoto is not spoken language, but written (litterary) language, so the 'quarrel' is retold in some way
So why do we need both ti and tu? I'd have expected one as a way of emphasis but not both?
Some verbs always use a reflexive construction (different in different languages). In Italian join is reflexiv, the verb unirsi (unire + reflexive pronoun)
No one has suggested how "you brought us together" makes a lot of sense!
Both are correct and like you said, the Tu is superfluous, like saying (Tu) mangi della frutta, because the verb shows the person doing (or who did) the action is Tu
.... however, the English version is incorrect. ' You united with us' rather than 'You united yourself with us' Duly reported
Not quite. In some languages there are reflexive verbs (like 'unirsi' here) that are not reflexive in some other languages (like 'join' in English in this case).
Some other verbs that are reflexive in Italian, which are not reflexive in English, are: ricordarsi, fermarsi, chiamarsi etc, such as in the following DL examples:
"Credo che lui si ricordasse DI lei."(= I think/believe that he remembered her)
"Mi sono fermato e ho aspettato che passasse la macchina." (= I stopped and I waited for the car to pass.)
"Sono quasi sicuro che (lui/lei) si chiamasse così" (= I am almost certain that was how he/she was called.)