So I did a run on a game, I did the obtain diamond classic. But Can I put it on /visit allowed as well, because it fills all the requirements of both the categories.

1. We have no idea what game are you talking about
2. Ask in the forums of that game, or ask the game moderators directly

Just in general, if you do a run, and it fits 2 categories requirements, can you put the single speed run in both the categories. (or do you have to do one for both even though that one fits both categories) also it might be different for each game, but idk I have not done a lot of speed runs.

Different games have different rules. It varies.

Generally that is allowed, though make sure to check the game’s rules.

Personally, I never liked this period for a lot of different reasons and the primary one that comes to mind is when people forcefully adds ILs to a leaderboard but it mimics that of how the any% or whatever is setup. So, then what some people end up doing and I used to see it from time and again was that they would splice up their any% run and submit that segment to the IL board.....its redundant. So now you've got an any% run on the boards with segements also cut out of it for the IL board.

Another time I once had a situation that someone tried to submit their Wario World any% run into a glitchless or no super jump category (I forget which, its been a long time) because their run ended up meeting those requirements. I didn't want duplicates across the leaderboards and would have people strictly stick to a single category for their run, I just felt it was pointless regardless if it did fit, I saw no point why anyone would want to see duplicates across the leaderboard. I believe we ended up trying to fine tune that issue so that wouldn't be an issue.

Although I know its different across every single game, it always seemed like the above two examples was what I generally ran across. Every category should be different and no same run should be able to go across multiple ILs/categories in my personal opinion.

I disagree on using full game runs in ILs personally. Well not every game will tend to this well, I think there are games where it is effective. e.g. Super Mario Odyssey Any% runs can and do end up on the IL leaderboard.

Generally, if the vod meets the requirements for both categories then yes, you can submit to both.