My MBP mid-2009 has decided to assign two functions to certain keys

I've never seen it do anything like this before. I am running Yosemite 10·10·3 and starting this morning, every time I for instance hit the Return key, it also will type a left bracket. Here's a list of all the strange things it's doing right now:

escape = escape + I`

mute = mute + opens System Preferences to the Sound panel

raise volume = lower volume

eject = eject + ;

delete = delete + lower volume

tab = types the word ‘to’ (?????)

p = p + caps lock

left bracket = -[

\ = changes Input Source to Romanji

return = return + [

/ = =/

fn = -

option = opens System Preferences to the Sound panel

0 or right-hand command key = command+0

left arrow = left arrow + return

right arrow = return

up and down arrow = \