if i do that, it will just show one ad from my sites and it wont display all the ads beacuse i have the same code, is that correct?
You can copy it and add it to as many sites as you wnat. It only shows that one channleso you do not know what sites are getting clicks. I would make a channel for each site so you know what site is earning you more money
but if you put an adsense code on one website you wont be able to place the same code on the same website. So one ad will show but the other one wont beacuse you are using the same code on the same website, but if you put it on a different website it will. am i correct?
You can put the same code on as many sites as you like, and up to 3 times on one webpage. If you put more than 3 on one page you're violating adsense's tos.
I was wondering about this earlier. Putting the same code twice on the same page(block units) thought maybe it would show repeat ads
I wouldn't do that... Try to create different channels for different sites, it's better to see which sites perform more. It's also good to make changes if the sites didn't perform well.