这段产生随机数的代码，怎样才能让随机数不重复？

<%
Function gen_key(digits)

dim char_array(50)
char_array(0) = "0"
char_array(1) = "1"
char_array(2) = "2"
char_array(3) = "3"
char_array(4) = "4"
char_array(5) = "5"
char_array(6) = "6"
char_array(7) = "7"
char_array(8) = "8"
char_array(9) = "9"
char_array(10) = "A"
char_array(11) = "B"
char_array(12) = "C"
char_array(13) = "D"
char_array(14) = "E"
char_array(15) = "F"
char_array(16) = "G"
char_array(17) = "H"
char_array(18) = "I"
char_array(19) = "J"
char_array(20) = "K"
char_array(21) = "L"
char_array(22) = "M"
char_array(23) = "N"
char_array(24) = "O"
char_array(25) = "P"
char_array(26) = "Q"
char_array(27) = "R"
char_array(28) = "S"
char_array(29) = "T"
char_array(30) = "U"
char_array(31) = "V"
char_array(32) = "W"
char_array(33) = "X"
char_array(34) = "Y"
char_array(35) = "Z"

randomize

do while len(output) < digits
num = char_array(Int((35 - 0 + 1) * Rnd + 0))

output = output + num
loop


gen_key = output
End Function


response.write "<pre>" & gen_key(13) & "</pre>" & vbcrlf
response.write "<pre>" & gen_key(14) & "</pre>" & vbcrlf
%>　
想让他生成的数组不重复，有什么好的办法？