welcome back antonio!   

not too sure about the oxidized part, but i think i noticed a similar situation as you when i left my keyboard untouched for about half a year, 1 or 2 of my keys don't register properly or don't register at all.
i didn't bother to open them up, i just tapped at it very furiously since i was playing game and kept losing because of it lol! (it's using an old WYSE black switch.)
then i just left it to collect dust again and then picked it up to use again after i got new keycaps.
for some unknown reason, it works again. 

i know the switch stem and the copper contact point tend to have slimy/greasy substance growing on it after a while as i have seen from most switches that i harvested from old boards.

double or triple register is not good >.<
i am surprised it happens to linear switches, i thought usually it happens to blue switches.

my guess is there's dirt inside interfering with the contact point, but if it's me, i'd just replace all the switch lol!

do you mean a metal case?

Shyt.. my MKB starts to suffer double-click issue... Press once = twice..

-  What is key chattering/bouncing?
Key "chattering" or "contact bouncing" is a natural occurrence in mechanical switches that can cause one switch press to be detected as multiple presses. Under normal switch operation the digital electronics in a keyboard will filter out a small amount of natural chatter/bounce, however, in the case that an individual switch is damaged or defective, the amount of chatter/bounce can become too large to be filtered. The result is a repetition of key input seen on your computer screen for only one intended key press. This is a problem that affects only the broken switch and is a problem that is covered by your EliteKeyboards warranty.

How to test for chattering:

Severe chattering is actually very rare, but it can be very frustrating if you think the problem is you when it is actually the key switch.

If you're under a Windows operating system, we offer our own testing program Switch Hitter for extensive chatter testing. Testing under other operating systems can be achieved using the following procedure:

1. Start by opening up a text editor like Notepad in MS Windows.

2. Repeatedly press and depress the suspect key and alternate each keypress with a space. It is a good idea to vary the force with which you hit the key in question while performing this test; as chattering can sometimes show up more frequently with hard key presses, but not with light; and visa-verse.

A good switch will have output like this on your screen:

d d d d d d d d d d d d d d d d d d d d d d d d d d d

A chattering switch will output:

d d d d dd d d d dd d d d d ddd d d d d d dd ddd d d

3. Repeat step #2 for several lines and also test a few keys you do not suspect as a control case in order to eliminate your brain and fingers from the equation.

How to fix chattering:

Please, under no circumstances should compressed air be blown into a mechanical switch!! This bad advice is repeated across internet forums, and it is a recipe for disaster. Please do not do this!

If you find you are experiencing a problem with one or more switches, please contact support@elitekeyboards.com.