If the data page is sourced from a report definition, you can configure the report definition to remove duplicates. If you have other columns in this report definition, then you may need to source the page from a different data page that only has the one column to ensure distinct values.
The check box to remove duplicate rows is in the Edit filters section of the query tab on the report definition.
If the data page is sourced from an activity, you would handle the removal of duplicates in the activity.
Another option would be to source the multi select control from a clipboard page, this page being sourced from the data page with the duplicates, removing the duplicates as you iterate through the data page and append to the clipboard page.