Sure, you can write more than once. Just keep it at a respectable amount, don't go over-sending (spamming) them. I have written to several celebrites more than once, whether it be because I hadn't recieved a response for a while, or I didn't like the photo they sent/I sent. You don't have to mention it in the letter, chances are even if they saw your first letter, they won't remember you. Good luck!
Although I have already recieved responses from all 4 of the following, I am about to resend to Mythbusters, Bear Grylls, Zach Galifianakis and Anna Kournikova, because I didn't like the photo's they sent, or I found a better send than the last one I sent.
Khackdrums
