This wasn't quite doubling. Five out of six frames were sampled at full 8-bit resolution, while the remaining frame was sampled at 7-bit resolution (hence the "robbed bit"). The "doubling" is probably that you could run 24 digital lines with the 7/8-bit encoding, over the wires used by just two analog phone lines.
Robbed bit signaling was (is) used to encode signaling information on a digital circuit (typically a T1 in North America) without a dedicated signaling channel.
There were a variety of multiplexing systems to get more than a single line on a copper pair, but they didn't have anything to do with robbed bit signaling.
This wasn't quite doubling. Five out of six frames were sampled at full 8-bit resolution, while the remaining frame was sampled at 7-bit resolution (hence the "robbed bit"). The "doubling" is probably that you could run 24 digital lines with the 7/8-bit encoding, over the wires used by just two analog phone lines.