Column A contains a list of values, each of which may appear more than once.
We want to create a list in column B in which each value from column A may only appear once.
Use the INDEX, MATCH, and COUNTIF functions as shown in the following Array formula:
The first cell in the unique list (column B) must remain empty.
The formula should be entered in the second cell and copied down until the #N/A error is returned.