Cancellation of vehicle insurance

I need intervention with my dispute with First for Women which covered my vehicle from 01/12/2020 but only debited on 15/12/2021. Now I'm unable to cancel because I'm expected to pay a portion of the premium on 15/11/2021 while I will also be paying full premium with my new insurer, as by debit date is the 15th. I tried calling them last week on 03/12/2021 and I was told they will call me back to discuss the review cover with me. They never did. Upon calling them again on 08/11/2021, I learned that I will be liable for a portion of the premium. I asked is it possible that my vehicle was insured by 2 separate insurers in the 15 days (1-14/12/2022), because by the 01/12/2020 I was still covered by my previous insurer? If I'm insured, do I not pay upfront before my vehicle can be insured even if it's a pro-rata rate? Should I need to claim, who will cover the claim? I will like clarity and assistance on the matter. Maybe I'm just really that dumb.