Deleting items in a list without duplication of what is left
 

I have created worksheets with drop down boxes linked to another worksheet
that I have called "data". The problem is, when the user chooses from the
list, let's say three items, and then decides they only want the first two.
When they delete the third item and press enter it add the first two items
again (so the first to items are listed twice)

For example if they choose red, green and blue and then delete blue it shows
up in the cell as red, green, red, green.

How do I get it to not double up what is left in the box? Here is the code
I used so they could pick more than one item from the list.

If Target.Column = 4 Or Target.Column = 7 Or Target.Column = 9 Or _
Target.Column = 6 Or Target.Column = 11 Then
If oldVal = "" Then

Thank you for any help.