Nope it is not harmful. Haha I know it is sick though. As long as he doesn't do it every single time I wouldn't worry about it. Worse case scenario, he might get a belly ache but I would think that would be about it. The reason he does that is because children that young don't understand that it is okay to not swallow it or put it back in them. They think that since it came from them that that's where it needs to be...if that makes any sense. I used to babysit a little girl who would spit up on the floor and try and hurry up and eat it off the floor because she thought she did something bad. It was soooo sick but kind of funny.
when our son would do this, we'd joke that he was "chewing his cud", like cows do. i don't think there's any reason to worry, as far as normal baby spit-up. unless food poisoning was the reason for the spitting up, there's nothing much in the food that's going to harm the child the second time around. gross as it might be. :)