First you have to create the formula (ex: =IF(MAX(COUNTIF(C$3:C$9;C3:C9))>1,"list is not unique","list is unique")
Then you have to convert it to an array forumula by pressing CTRL + SHIFT Enter

If you want create an array formula in excel, you type the formula into a cell and then push CTRL, Shift and Enter at the same time. into the cell show you #value if you did wrong.

the Excel COUNTIF formula/function can only check to see if specified cells meet one condition,
COUNTIF Syntax
=countif(range,criteria)
=COUNTIF(A1:A20,">20").

You can use a function like that: =COUNTIF(range, criteria). If you want to count several columns and put the result in an array, you can drag the function down with the mouse.

To create an array formula in excel you type the formula into a cell and then push CTRL, Shift and Enter at the same time. If you do it incorrectly Excel will display #value in the cell