I agree with child_like empress: the film is all about duality (check the use of mirrors throughout the film). We also find out that Jack has been in the hotel before, too (he was at the 4th of July party in the early 1900s). So Delbert and Charles Grady are really the same person, and the use of different names is not a mistake. It's another Kubrickism to make you go hmmmmmm....
Yes, I agree. The movie's main concept is duality. Just like Jack, Grady has two different personalities. Delbert Grady is trapped in the past (July 4th, 1921), and Charles Grady is trapped in the winter of 1970, when he killed his wife and twin daughters.
I disagree with this. There can't be two Grady's, since Jack mentions upon hearing Delbert Grady's name that he was the caretaker, and it ends with he (Grady) telling Jack that he "corrected" his family. Clearly this is the same Grady that Ullman was talking about.
Isn't the simple answer that Kubrick screwed up and mistakenly called him "Charles" in the opening scene, when he was supposed to be called Delbert? He is consistently called Delbert throughout the original King novel, so are you suggesting that Kubrick snuck some secret meaning in the script? Is it not more likely that an overworked director just had a name slip by?