» Retrieving Unique Values from a List

Problem:

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.
Solution:

Use the INDEX, MATCH, and COUNTIF functions as shown in the following Array formula:
{=INDEX($A$2:$A$8,MATCH(0,COUNTIF($B$2:B2,$A$2:$A$8),0))}

Note:
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.