Populate a unique list from two columns- array formulas to slow!

Hi Excel experts,

I am currently using array formulas to populate a unique list from two columns on different worksheets.

The two columns that the new column is drawing from might be added to or changed so I need it to work dynamically.

The formula I have is...

Formula:

=IFERROR(IFERROR(INDEX(SKU_Col,MATCH(0,COUNTIF($C$1:C1,SKU_Col),0)),INDEX(SKU_Col2,MATCH(0,COUNTIF($C$1:C1,SKU_Col2),0))),"")

This does the job but it is seriously slowing down my spreadsheet... originally populating the list after dragging the formula down (about 3,000 rows) took almost an hour!!!

Any solution would be greatly appreciated so I can get my spread sheet running again.

The more automatic it can be the better though.

Thanks