Does courage stack?

I have an assassin using black and I get courage in 2 hits, so it is less than 8 seconds. If I cast courage while another one is still up does it just reset the timer, or does it get a bigger dmg boost and stack?

 

If I'm not mistaken each activation of courage counts as a separate timer, but yes, they do stack.

So, for example, if you cast it once and then cast it again a few seconds later, then you'll have both effects on at the same time - resulting in much higher damage - until the older one expires, at which point it goes back to being the standard 150% extra damage.