Seem to have come across a weird glitch that's interfering with my HUD, i.e. the life bar has a black square obscuring the top, and the lower half is repeating for some reason. Tried rebooting and it's still there, so I assume there's nothing I can do, but I thought I'd ask anyway.

I don't see anything wrong. Are you talking about the upgrades? The top part (part I think you're calling a black square) is space for the health upgrades for physical attacks, and the bottom part (which you think is repeating) is the health upgrades for bullets. The upgrades act as an extension to your health, with one of them protecting against physical attacks and the other for bullets. Just read the corresponding upgrades in the Waynetech tab and it should be clear.

Yeah, I just got the combat upgrade and now there are two segments. My bad. I just wasn't expecting those to be integrated into the HUD for some reason.