Discovery Refuses to pay back premiums.

I was unaware of an extended warranty cover that i had with my previous insurer... That policy was cancelled and then i took out extended warranty cover with Outsurance... where i actually had a claim on extended warranty over... I then moved my insurance to Discovery insure with Extended warranty cover... When i found out a month ago that i still have extended cover with my first insurance company before outsurance, I wanted to cancel my extended warranty cover with Discovery and backdate my premiums as i was not aware of this duel insurance... All the proof was send to Discovery showing that policy was cancelled and i was not aware of the dual insurance... i mean I even had a claim with my second insurance company for Extended warranty... All i want is my extended cover to be cancelled and back dated premiums. All the proof was send as they requested... but then they denied to do the refund.