If you're borrowing someone's car, with insurers like Cuvva, you'll get an insurance policy in your name even if they already have insurance in theirs. It's legal to get multiple policies on your own car in your name - but there is no benefit to it, unless you wanted to temporarily upgrade your insurance for a short period (for example, getting fully comp insurance over third party). Insurers won't pay out twice for the same accident even if you have two policies on the one car.