Shenron was created by Kami when he arrived on Earth. After Kami fused with Piccolo, Shenron was revived by Dende.
Shenlong cannot bring back a person from the dead more than once. Shen Long can originally only grant one wish, but after being revived by Dende, he is able to grant two at a time. Shenlong often threatens to kill the summoner if they do not quickly make their wish, but never follows through with it.