My laptop keyboard outputting multiple key presses

If it is new enough that it would now be under warranty, this should not be occurring, you should seek warranty coverage, because the fix is probably a replacement keyboard. So, since you say you "voided the warranty" we need to know what you did that you think voided it... because what the company says and what the LAW says about that are often different, In the US for example just breaking the "seal" IS NOT voiding the warranty, in fact. The law says that DOES NOT void the warranty. SO what did you do? You WILL have to fight them on it, but depending on what you did, you may win. SO... what did you do to it?

@uchetil

The problem happened before I opened the laptop btw, I upgraded the ram from my old laptop, I heard that in certain regions (I live in the Philippines) that it's fine as long nothing got damaged in the process.

I'm not an expert, but my understanding is that a user upgrade of the memory should NOT void your warranty, I can't say with certainty that that would apply in the Phils, though. Worst case scenario I'd check for a second hand or new keyboard, I just did a search and found prices varying from about $30 to $60 USD, that's for NEW red backlit keyboards for your model. I'd advise at least trying for the warranty first though.