Avoiding Duplicates

I have a string value which has the bellow content.

Dim aString as string

aString = "1,2,4,2,2"

where aString is a parameter set dynamically based on the user loggin into the system. Now All I want to do is to eliminate the duplicates. ( In this case I need to keep only one "2" and remove the remaining "2" , so my final string should be "1,2,4" ).

How could I do that. I was thinking of an array, but the issue is, If the "aString" is empty I believe array will return a error. So any other way of doing ?. Or simple asp functions which will eliminate this.