I had created a Speedrun board for a game called "The Mast" but only to just now notice that there already was one, im not sure what i should do

What's the link to the duplicate game?

Your game seems to be the newest one added, and I assume it has the least amount of runs.

So I would message the other board's moderator(s) and try to get yourself added as a mod onto their board, so you can moderate it with them.

If added, then I would resubmit the runs that are on the duplicate board onto the non-duplicate one. On the game you mod, there are currently only 4 verified runs, so this wouldn't take too much time. And this means that no runs are lost.

Afterwards, I would submit a support ticket, requesting for the duplicate board to be deleted.

Also if you don't want your board deleted, then try to come to an agreement with the other game's moderator(s) on what to do, like with what cateogries you should keep, and with which game to delete.

I should add that if you do talk to the first board's moderator, they should by no means be obligated to add you, just because you weren't initially able to find the existing board (I realise it's hidden in 'unofficial' games, not to worry, but even so...) - to be honest I'm not sure if there's any site policy on what happens here because games are almost always checked carefully before being added to the site, can't recall this happening, but having one leaderboard is quite important ':D

Fully concur with Yummy Bacon that you should try and contact the first mod, merge the runs into one board, and then request the newer board is deleted.

site policy on what happens here because games are almost always checked carefully before being added to the site

No, I've seen people say that site staff have fallen off recently as they've seen more instances of duplicate games getting added.

I don't believe there's any written down policy anywhere, but I assume they try to be consistent when stuff like this occurs. And it seems that, most of the time, the games' moderator(s) should try to sort it out.

Although, you could always message site staff about this if the other moderator(s) don't help or don't respond, etc. If you were gonna message them, then message starsmiley as she does game requests.

they've seen more instances of duplicate games getting added.

That's a shame, I hadn't previously noticed duplicates (except I think on one or two occasions where a series moderator added something which already existed elsewhere). Part of me hopes there's some kind of explanation to duplicates cropping up other than carelessness, and that it's not for us to see, but hey.

Obviously you're right consistency is very important; having the two moderators come to a decision is ideal, even if by default I personally would hope to see the original board given the space to work in (especially if there's an active community around it, although this could become true of a new board if the mistake somehow went unnoticed for a while).

When I've seen duplicate games added, I've reported it in the support hub and the duplicate usually gets removed pretty quickly. In one case where I wasn't certain, I contacted a moderator of the original board and let him make the report.

It's still better for the different mods to work together so runs don't become deleted.