I was doing some work in an estate agent and found a socket that would not turn off, even when whole DB was turned off. It turned out to be fed from the shop next door, a mistake when the shop was split into two units.
But over what timescale are you talking of the electricity price doubling, because over the last say 10 years it HAS doubled in price.
Check the readings over the last 4 quarters for instance, see if the demand has risen. Have the billings been read or estimated, the supplier can get away with reading just once a year and estimating three quarters, they have been known to increase a direct debit on estimated readings.