I got bored :)
if you download the excel file below and open it then it will give you a random sequence of numbers 0-9 and letters a-h in the yellow cells. if there are duplicates in the sequence it will tell you and then you just have to press F9 to recalculate until you get a sequence without duplicates
The numbers and letters in the list are taken from column A so if you want to add more letters just change anything in column A and it will be included...
http://s000.tinyuploa...=37823782559651221613
If anyone can figure out how to make it recalculate until it gets a sequence without duplicate please go ahead :)