There may be an easier way than this....but here goes:
I'd create a custom view in the Lead Database which just shows that one field. Then I'd export the entire database into Excel (making sure to only use this particular view). This will export just that one field value for every single lead, and the Excel file should just be one long column.
Then open the Excel file, and you can "Remove Duplicates" from that column. What you're left with is a single entry for each unique value.
Hope that makes sense. I'd be glad to clarify if needed.