When reapplying sunscreen

I make sure to wear sunscreen as a must these days, but I've noticed some people say they reapply it. I'm wondering if it's better to wash my face thoroughly before reapplying, or if it's okay to just apply it directly over the existing layer. Since I sweat a lot, I've never even considered reapplying it right on top of the current one. I'm suddenly curious about how to properly reapply it outdoors and whether it's okay for my skin to just apply it directly over the existing layer.