******* to signing a contract and bodylab employee filling in contract duration in my absence

Bodylab employee said, body lab is giving one month vouchers so that people can go to their gym and after that decide whether or not they want to get a contract and carry on, I filled in the form for me and my friend because he said I can invite my friend and I signed, that body lab employee said I mustn't fill the period. After a month I received a call telling me that Im owing body lab, I went to the branch durban field Street to query and they told me I signed the contract, I asked if I can see it, when I looked at it I saw that the gym/ contract duration was filled in, I left that section blank and it is evident that it was filled by someone else because the hand writting is not the same as mine. I paid the cancellation fee for the second person on contract, later I moved from town I cancelled the contract and I payed for this too. This was in 2014 and today I received an email from debt collectors telling me that I must pay 5000+ .I payed for this contract to be closed, how many times must one pay to cancell the contract? What body lab is doing is not fair, from the way their employees are( getting people to sign contract by telling them lies). From 2014 why were they quite all this time, we're they waiting till I loose proof of payments so that they can ask for another payment. This is totally unacceptable.