2 characters appear when I use the 's' 'a' 'w' '2' keys!

I have had this same problem more than once now. I just fixed it by pressing on all the keys which were not working at once and holding them down. In my case, the keys were 2, 3, z, e, s, d, x, c. (I am using a French AZERTY keyboard, so all these characters are in sets of two, cascading down from the numbers.) It was a very simple fix for me, after a long time of trying other ideas, like resetting SMC and PRAM.

I hope this helps other people if they find themselves in this same situation!

Hello PippaG,

Thank you for using Apple Support Communities!

It sounds like the keyboard is pressing extra keys when you press S and W.

I would first check to see if it happens in a Safe Boot

Safe Mode is the state into which you can start up your Mac by performing a Safe Boot. To perform a Safe Boot, press and hold the Shift key immediately after your Mac starts up. Continue holding down the Shift key until a gray Apple logo appears on the screen.

From: Mac OS X: What is Safe Boot, Safe Mode?

http://support.apple.com/kb/HT1564

If it does I would next try resetting the SMC and the PRAM on the computer.

Intel-based Macs: Resetting the System Management Controller (SMC)

http://support.apple.com/kb/HT4721

About NVRAM and PRAM

http://support.apple.com/kb/HT1379

All the best,

Sterling