I thought that helfen required Dative without any preposition, but in one of my favorite songs by J.S.Bach, the lyrics go like this:
Wir eilen mit schwachen, doch emsigen Schritten,
O Jesu, o Meister, zu helfen zu dir!
...
Is this usage normal? Still used today? Does it have the same meaning as without zu? Or was the zu just stuck there for meter's sake?
The grammar is unusual, but I believe it's valid.
Bach chose to leave out the dative object of helfen altogether, only to imply that it is Jesus himself the speakers were coming to aid.
Written in its long form using standard order, the sentence reads:
Wir eilen mit schwachen [...] Schritten, O Jesu [...], [um Dir] zu helfen zu Dir.
Zu Dir indicates the direction of movement (eilen), and is independent of the purpose of movement (zu helfen).
Finally, (Um) Dir is the missing dative object which belongs to helfen. In an indicative sentence it would be "Dir", for instance:
Ich helfe Dir.
Since the sentence uses final clause ("Infinitiv mit um ... zu ...") we need to add "um":
Ich komme, um Dir zu helfen.
No. You misunderstood that the 'zu' is part of the 'eilen'. The 'helfen' has no object, it's just saying they are 'hasting to him to help'. So it's the same as in English.
In reality, things are much more simple. The punctuation is not in the original. One should read: "Wir eilen ... O Jesu, o Meister zu helfen, zu dir!"
The words 'Meister zu helfen' (= A master when it comes to help) was a kind of title for Jesus, derived from the Old Testament and very common in sermons and meditations in Bach's days (and long before in christianity)
