Hi ,
n
I don't think you can remove duplicate values in this scenario. Because, for a list item, the value saved in a lookup field is not the display text, but the link to the list item in the parent list. For example:
n
n
Though the display texts of the items are duplicate, their links are unique. They are actually not duplicate values.
n
My suggestion is create a new list and add a "Type" column. The values in the "Type" column are distinct values. Use this list as the parent list of the lookup field.
n