I learnt about cp's -n (or "no clobber") option that lets you copy, but not overwrite files that already exist:
cp -Rvn folderA folderB
The above would Recursively copy the contents of folderA into folderB, and print a message per file it copies (with the v option). Great to know when things get stuck or interrupted :-)
It's quite useful when trying to copy from something like a folder in a remote drive that keeps disconnecting. So if the network times out you can always run the command again after reconnecting, and it will skip files that are already there.
Note that if a file has been partially copied, then cp will skip it the next time and not complete the download. So don't use this method for stuff that requires some degree of integrity--you'd better reach for tools such as scp.