It would make sense to contact the supplier of your phone's insurance and inform them that you had found your phone and see what happens from there i guess.
If your insurance company provides a new phone, or pays for replacement, and you then find the phone you lost, you need to immediately call the insurance company. Legally the old phone belongs to them (since they paid for its replacement), but the policy may call for specific remedies in such circumstances. Only your insurance company can tell you how to proceed.