I always thought it was "here, here" meaning I too, me standing right here, agree with what you say. I did so because of the way it is shouted out - loudly, with no hesitation between each word.

When you want someone to pay attention you might say "hear, hear" but wouldn't you pause for effect and to draw the person's attention after the first "hear" then say the second "hear". I'm having a bit of trouble explaining it. Act it out and you may see what I mean.