Answer: If the first Azhaan that was given prior to the normal scheduled time, took place before Zuhr time could even commence (in other words prior to the lapse of Zawaal), then that first Azhaan was void and it was incumbent for Azhaan to be given again after Zuhr time had come in. If this was the scenario, then the repeating of the Azhaan a second time was in order.
If, on the other hand, the first Azhaan was given after Zuhar time had set in, and it was given openly on the mic system, it was sufficient and there was no necessity to repeat the Azhaan. Notwithstanding the fact that the second Azhaan was unnecessary, it did not in any way affect or taint the validity of the Zuhr Salaah that was performed subsequent to the two Azhaan’s having been given.
However, if the first Azhaan had been given discreetly (without the rest of the congregation being aware that Azhaan had been given already), then it would have been in order if a second Azhaan were to be given over the public address system according to normal routine. (Ref: Fataawa Aalamgiri, Vol. 1, Pg. 53).