Those who leave the service are advised that they must look after their discharge certificate as duplicates cannot be issued and they are required for re-entry.
Most people "losing" their discharge papers generally do so for good reason or simply would rather someone else looks for them on their behalf.
Fortunately there is a method of obtaining copies of data held by the service & it is up to the individual to follow this route of enquiry to provide evidence of their former service. Sadly it's not a simple one-off phonecall, it involves writing a letter to the RN Data protection cell (details available from your AFCO) & then dwelling a pause of several weeks to see what, if anything, they can turn up for you.
It's probably worth remembering that any seniority earned in service is unlikely to be awarded upon re-joining unless you can provide proof of earnings via old pay statements.
Good luck in your quest, you're the second to ask this very day!