If they have told you its x2 for the work canvassed, its x2. But unless they have said that for bi-monthly you will be charged 1x, then that is wrong for them to charge you x2.
What does the company you use, use for customer details? I get a signed copy of what has been agreed with the customer, ie customers details, frequency of the clean, what i have to clean, price, ect ect. I get a copy, customer gets a copy and copy for canvassing company.