Comparing Lists for Duplicates

Imagine for a moment that you have a worksheet that contains lists of part numbers. On one worksheet you have a list of part numbers, and on another worksheet you have a similar list. The lists are not identical, however, and you want to determine if a particular part number on one list also appears on the other.

One solution is to somehow combine the lists, but add some sort of indicator as to which original list the particular part number came from. This approach (or a variation thereon) is, in fact, the approach taken by many Excel users.

What if you don't want to combine the lists, however? In this case, there is a very easy way to do the comparison. Follow these steps:

1. Make sure there is a blank column just to the right of each list of part numbers on each worksheet.
2. Select the part numbers on the first worksheet and give them a name such as "PartList1". (In Excel 2007 display the Formulas tab of the ribbon and then click on Define Name in the Defined Names group. In older versions of Excel use Insert | Name | Define.)
3. Select the part numbers on the second worksheet and give them a name such as "PartList2".
4. Assuming that the first part number on the first worksheet is in cell A2, enter the following formula in cell B2:

     =ISNUMBER(MATCH(A2,PartList2,0))

1. Copy the formula down so that a copy appears to the right of each part number on the first worksheet.
2. Repeat steps 4 and 5 on the second worksheet, but use the following formula:

     =ISNUMBER(MATCH(A2,PartList1,0))

When you are done, either TRUE or FALSE will appear to the right of each part number on each worksheet. If TRUE appears, the associated part number appears on the other worksheet. If FALSE appears, then the part number is unique and does not appear on the other worksheet.

Another approach is to use an array formula to do the comparisons. You could follow the same steps shown above, but use the following formula in step 4 (and PartList1 variation in step 6):

=OR(EXACT(A2,PartList2))

Since this is an array formula, you would enter it by using Shift+Ctrl+Enter. The result is the same TRUE and FALSE designation described above.

Regardless of which formula approach you use, you can use the AutoFilter capabilities of Excel to limit what is shown on either worksheet. If you filter to show only the FALSEs, you will have a list of all unique part numbers. If you filter to show TRUEs, then you will have a list of duplicates.