You have a couple options, I'll list them in order of how "ethical" they probably are:
1. Build a cabin and get another player (or yourself with a controller or launching the game twice) to catch it - downsides are that you need at least a few levels of fishing, you can use buffs and a master rod as well as the fishing buff from the desert festival to only need a couple levels though, still going to take an hour or two
2. Edit the save file to remove the legendary 'check' which makes you unable to catch another - this requires a guide or some experience to find the right string of code, there is a guide here on the forums from a while back that Nimeld posted
If you use this method BACKUP YOUR SAVES
3. Download a mod which allows for multiple legendary fish - not possible on most platforms and may take even longer than option 1 if you aren't already a bit experienced with modding
4. Use an item ID to spawn it in, there isn't a long enough name box to have quality included in the name check (I think but I'm not to experienced with 1.6 name injection so I can't really tell you for sure) but you could easily have a plain quality legend which looks the same in a fish tank - definitely the fastest though most questionable (though I will argue, you already caught it yourself and aren't going to sell it or anything so I'd say it doesn't matter)