If you already have a website with ads and an adsense account and you have another website thats over the same type of content can you just re-paste the same add code or do you have to re-apply for each website you post the adds on ?
All you need is 1 account you can use adsense in other sites you own make a channel for it and add to allowed sites just make sure you follow the tos on all sites.
I think you could...but I would suggest generating new code geared towards the other site. That way you can customize the colors and make it fit in with the new site. Also, if you have new code, you can give it it's own channel so you can monitor its progress.
No problem, I think you can use your adsense account on as many sites as you have. But copied contents I am afraid won't bring you higher ranking and good traffic. So you won't have high click though rate and get paid less.