There is a button to open a pop-up. When the Pop up closes , it refresh the parent page (Refresing the parent page is necessary to get the pop up data into the text box which is also there in the form).
Now when the page refresh.. the selected items of shuttle duplicates.
What is duplicated, the left or the right side of your shuttle? If it's the right side, have you checked that the code to populate the shuttle page item value doesn't do any concatenation. Check that he page item source is set to "always replace existing value".