preventing entry duplication in string

Hello, I have a page of checkboxes.
When each checkbox is clicked I want a unique value entered into a string. The string should build up as multiple checkboxes are clicked (the string wil then be submitted to coldfusion for a database search).
I set up the below, however the "+=" operator duplicates previous string entries. How can I overcome this duplication ?
...or perhaps I have gone about the wrong way to perform this ?
Thanks in advance for any help.

RELEVANT CODE SEGMENT:
var selections:String = "";
myCheckboxListener.click = function() {
if (checkbox1.selected) {
selections += "tanks";
}
if (checkbox2.selected) {
selections += "aircraft";
}
}

submitbutton.onRelease = function (){
trace (selections);
}