keyboards are 2 layers of plastic film with tiny electric contact, separated by a third layer of plastic film with holes on the spot of every single key.
when you press a key, the top and bottom plastic are pressed against one another inside the hole of the middle layer, and the electronic signal going from the top to the bottom layer tells your computer that the key is pressed.
problem is: separation is the same as an average good quality book paper sheet, or maybe the same as the aluminum sheet that makes a soda or beer can. also, the electricity that goes trough your keyboard is extremely tiny, and sensors are VERRY sensible, so your air battery can lasts longer.
So even if you have a single hair touching those circuits, your keyboard would "break" the same way it breaks when you spill coffee or whatever liquid; even after the liquid dries, the coffee and the sugar are probably still there "pressing" your alt key.
solution: (this will probably void any warranty) open your computer carefully, remove the keyboard, dismantle all keys one by one, separate the plastic layers with surgical precision so you don't cause any ruptures to the electric "wires", then wash everything with a lot of water and very little mild soap, take extra care to remove ALL the soap, leave everything to dry in the sun (you know electronic components don't like humidity, right?) assemble everything together with "just-washed" oil free hands (don't touch the inner part of the sensors now).
Doing all this, if you have any luck your keyboard might work 100% again... or maybe you might make it stop working entirely, who knows? =)
if you have some money, then backup you computer, go to the applestore and tell them you want to buy a new keyboard, because you broke the current one.
they shall give you a brand new (refurbished?) one, and charge you only the new keyboard. or they might give the same one back to you, who knows? haha.
Best Answer
It has nothing to do with your build in keyboard. What u have encounter is due to MacOs Catalina. I have same problem. It started from bottom row of alphabetic of your keyboard. You get multiple Z after pressing the Z button once until you have to press the return key to stop it. It sorts of like a virus escalated to the rest of your buttons on the keyboard. Slowly all your small caps alphabetic auto Caps Lock although you have not activated the Caps Lock and numerics auto change to symbols without pressing shift button. The WORST OF ALL COMMAND button + R button (have been remove from your drive) after the update to MacOs Catalina that does not allow u to access to internet recovery utilities. Further Damage done by Catalina is that many apps not be able to support. So congrates! All u could do is to restart your macbook pro whenever should these problem arise each time.