I had a keyboard that i spillt on before and got the same problem. It was a cheap one. When i opened it up there was a kind of keyboard circuit board consisting of two plastic sheets put together with exposed flattened wiring in between that registered the key pushes. Water had gotten between the two plastic sheets which connected keys making it send the signal for more keys being pushed. It had gotten stuck because the plastic sheets sealed the water in so it wouldn't dry. i managed to pry open them enough for it to dry of and it still worked. A year later same thing happened but this time it didn't fix the problem. Looks like when i pried it open it i might have damaged it to much or something. You might at least wanna open it up and look if you can see the same kind of circuit as i had.

Might add this was over 10 years ago. Much might have happened and the circuit might not even be the same.Looked something like this.
keyboard_1resized.jpg
Keyboard_Construction_Button_Press.JPG