I saw some runs where they were at 30fps and others at 60fps, so I was in doubt which one is allowed because I thought it could only be at 30fps.

I think it's just because some people are just using the "Show Display FPS" option which shows the refresh rate that DuckStation is running at which will be 60fps/60hz if it's an NTSC game. The game internally is still running at 30fps which you can see if they use the "Show Game Frame Rate" option. Ideally people would show both. Either way the game should be running at 30fps.

if you check in the rules for the Duckstation category. you should find a description for the required settings. you should be averaging between 20-30 in regular gameplay but in doors and menus you should be hitting just under 60 fps.

These guys beat me to it, but yes the game run at 30fps. The 60fps that you're seeing is because of Show Display FPS being checked and not Show Game Frame Rate being checked. Seeing this thread and reviewing the wording of the rules it only says to have "Show FPS" checked, but not which one. The rules will be updated to include having both "Show Display FPS" and "Show Game Frame Rate" checked so this kind of confusion doesn't happen in the future. Apparently when Duckstation first came out there was only one option that would display both, but they have since been separated to those two options.

Yea, that's not me not noticing and updating the rules earlier. I think they made an update to Duckstation which split the two FPS options at some point, because I'm pretty sure I would've included both as I did for Survivor.
(edit)
Duckstation setup instructions: https://www.speedrun.com/residentevil/guide/lldhu