Happened to me, A Neighbor was registering a new telephone line, She lives at 44 I live at 34. They issued a new connection at my address!
The worst thing about the whole situation was 1 week prior to my line being cut I had a letter from BT saying there been a new line request at my address and its going through.
So like you do I phoned them up to inform them of there mistake and not to cancel or issue a newline. The guy on the end of the phone assured me that its been canceled, sorry for the mistake etc blah blah blah.
yet 1 week later BAM! disconnected and re-connected on a new number. Took just under 2 weeks to sort including re-activation of my broadband.
We complained and BT paid for my broadband activation and 1 months broadband.