How do you replace duplicate values in an array?

I'm trying to create an array (1 by n) filled with random integers. All of the integers have to be unique. I tried using unique but that only got rid of the duplicate values and shortens the array; I want to keep my array at a constant size. How can I replace these duplicate values, without affecting the others?

example:

[ 1 3 5 5 6 7 9 9 ]

turns into

[ 1 3 5 8 6 7 9 2 ]

or something like that (5 and 9 are replaced).

Or, better yet, is there any random function that doesn't repeat at all? Thanks.

Answer 1

[José-Luis]

 randperm(9,9)