deleting repeated text from a drop down box
 

I have drop down boxes that allow multiple choices. Here is the problem, if
more than one is chosen from the dropdown box and then you decide I only want
one and not the other and you go up to the formula bar and delete the text
you don't want and push enter it doubles the text in the cell. So, if I
choose green and blue and then don't want gree and I go to the formula bar
and hightlight the word green and delete it the cell with the drop down box
will now contain green, blue, blue

How do I get it from repeating text?