Do you mean kind of like geminate attack from dota?
As far as I know, you'd have to trigger it.
EDIT:
I did make a "double-strike" ability in my map that basically makes the hero hit again (does the same damage as the previous attack and applies all on-hit effects again). It doesn't have an animation component to it, though. It just does the damage and effects.
I had an idea for melee heroes where you could play their current animation with negative speed (basically rewind their attack animation) and then play it with positive speed again. With proper delays and speeds, you could make it look like they hit twice quickly, but it might not work for all attack animations (if they are too short) and it I don't know what would happen if you interrupt the animation. You could maybe pause the unit while it is happening.
For ranged heroes, it would be a lot more complicated. Easiest solution would be to just spawn another projectile. I don't know how you would detect what model to use, though.
All in all, I found it was going to be a ton of work for not that much gain. There'd probably be bugs littered throughout as well, so I just opted to make it do an invisible "hit".