wondering about this for a while. If ds (2 and 4 months )hits a child he has a sort of time out as a "punishment". We do exactly the same if he hits a child who has hit him first. I thought this made sense in terms of consistency ie we don't ever smack, but when he hits first I notice friends hardly ever discipline their child for retaliating. ooh the ettiquette of blinking whacking is so annoying.
I would but not as strongly as if they hit first, and would also try at explain (at a basic level) other things they can do e.g. tell mummy/adult etc.
I think you are right in terms of consistency. I also think they need to learn that it is not alright to hit even if someone hits them first, but some leniency required because it is hard to control emotional outbursts as a small child.