If you retire and have cobra insurance from a previous employer and get a new job with insurance can you still keep your cobra insurance and which insurance is the primary insurance?

COBRA insurance is a guarantee that your health insurance is portable when you leave the job. It is now private insurance where you are paying for it. The cost is probably higher than the employer provided insurance so you would want to look at the 2 carefully and determine which would be most economical. It does not make sense to pay for two policies. If your new job pays for (some of) the insurance dump your COBRA.