Clocks

I had a thought while in the shower this morning (and no it wasn't "How did I get so
stanky"). I was thinking about the guy who built the first clock. Why did he opt for a dual 12 hour time scheme instead of a 24 hour one? A day has 24 hours right? The military uses the 24 hour format as not to confuse one in the morning with one in the afternoon. It just makes sense; one in the afternoon IS actually the 13
th hour. So why would it occur to someone that we should have two One
O'Clocks?
We're used to it now and it's ingrained in our psyches and our culture. But when people first started using devices to measure the passage of time, before it became second nature; why two One
O'Clocks and not 13 O'clock?