Double Block

Certainly it is not the choice of the signallers but rather something that the signalling forces them to do. The likelihood is that in the situations you are thinking of, the last signal is closer to the junction than might have been the case on Network Rail meaning that the junction is well within the overlap and the additional protection of another signal in rear has been deemed necessary. Having said that not all the junctions you encounter on the Met have two signals protecting them. Harrow North Junction on the northbound (down) is protected solely by signal JB7. Perhaps more interesting is Watford South Junction. When I was a qualified signaller at Rickmansworth that junction, on the northbound (down) was protected by a single signal JJ48. A few years ago the junction was relocated further south (i.e. closer to signal JJ48) and it was deemed necessary to provide additional protection by making the signal in rear of JJ48 a controlled signal also. Thus the station starter at Moor Park became signal JJ48^A while JJ48 became JJ48^B. Now because the two signals carry the same number, being differentiated by the A or B suffix, this means that they are both controlled by a single lever in the Interlocking Machine Room and thus both clear together. The signaller cannot clear one without clearing the other. Neither can be cleared while the junction is set for a train crossing from northbound local to northbound Main and it is only track occupancy by the train ahead which could create the situation where one of the two signals is clear but the other is not.