While working with large Excel worksheets or consolidating several files, you may need to check for duplicates. Depending on the task at hand you could be faced with a wide variety of situations. This article describes the most common examples you may encounter and how to find duplicates in Excel.

If you are looking for a different approach, I have also posted an article on How to find duplicates in two excel sheets.

## How to find duplicates in Excel

Conditional Formatting is a very versatile tool. It has pre-defined conditions but you can also use formulas for more complex tasks. You can find the Conditional Formatting in the Home tab, under Styles section. If you select Highlight Cells Rules/Duplicate Values, Excel will highlight all cells which are found at least twice.

Please keep in mind that this approach works best when you a unique identifier to distinguish between different database entries.

Our example file contains a database with users. When they registered, they provided their First name and Family name and the system generated an ID. Now, let’s assume that we used several sources to create this table in Excel and some of the users appeared more than once. Using the Duplicate Values option on column E, we can highlight the users with the same name.

However, looking closer we can see that there are some users with the same name but different IDs. This means that using the pre-defined “Duplicate Values” Conditional Formatting is not good enough in this case. Simply deleting the highlighted values based on the name criteria would result in losing data.

A better approach is to check the full entry (ID + Full name) by creating a Conditional Formatting rule using a formula. To do so, simply select the area where you want to apply the Conditional Formatting rule. In my case this is range `$B$3:$B$202`

. Then, navigate to the “Conditional Formatting” tab and click on “New Rule”.

Choose “Use a formula to determine which cells to format” as the Rule Type and insert the formula you want to use. Don’t forget to change the formatting of the cells in the Format tab. My rule contains the following formula:

`=SUMPRODUCT(($B$3:$B$202=B3)*($E$3:$E$202=E3))>1`

##### How SUMPRODUCT formula works

In case you are not familiar with SUMPRODUCT formula, I strongly recommend taking the time to learn it. I assure you it has a lot more uses than to look for duplicates in Excel.

SUMPRODUCT returns the sum of the products of ranges or arrays. While this may not sound like much, keep in mind that SUMPRODUCT is a versatile function and can be used in wide variety of situations. This incredible function can be used to count or sum items based on criteria (like COUNTIFS or SUMIFS), but provides a lot more flexibility.

Syntax and arguments of SUMPRODUCT:

`=SUMPRODUCT (array1, [array2], [array3], ...)`

`array1`

– The first array or range to multiply, then add.

`array2`

– [optional] The second array or range to multiply, then add.

`array3`

– [optional] The third array or range to multiply, then add.

While SUMPRODUCT function uses arrays, it does not require the standard array syntax (Ctrl + Shift + Enter) to enter. SUMPRODUCT will behave exactly like SUM function when only one array is supplied. Keep in mind that SUMPRODUCT supports up to 30 arrays.

SUMPRODUCT assigns a value of 1 when the criteria is met, or a value of 0 when the criteria is not met. Then, it sums all the values and returns the total. Using “*” sign in the formula makes the two conditions cumulative.

##### Understanding the Conditional Formatting formula

The easiest way to understand how to find duplicates in Excel with this conditional formatting formula, is to split it into three parts.

Part one and two check to see if cells from arrays `($B$3:$B$202)`

and `($E$3:$E$202)`

match the values from cells B3 and E3 respectively. If they are both true, then it will return a value of 1.

Part three checks if the sum resulted from SUMPRODUCT is greater than 1. The conditional formatting formula will then move to the next set of cells, B4 and E4, and perform the same checks, then continue all the way down to row 202.

A simple way of looking at this formula would be:

a) SUMPRODUCT will check if ID from cell B3 is found in array $B$3:$B$202 and generates an array with only 1s and 0s: `{1, 0, 0, 1, ... , 0, 1}`

.

b) SUMPRODUCT will check if the Full name from cell E3 is found in array $E$3:$E$202 and generates the second array: `{1, 0, 0, 0, ... , 1, 0}`

.

c) SUMPRODUCT will multiply these two arrays and return the sum: `{1*1 + 0*0 + 0*0 + 1*0 + ... + 0*1 + 1*0}`

.

d) The formula will check if the result returned by SUMPRODUCT is greater that 1 (remember our “>1” condition).

If the value is greater than 1, this means that the combination of ID + Full name is not unique in our table.

*Note: Keep in mind that all the arrays in the SUMPRODUCT formula must be the same size. Otherwise, SUMPRODUCT will generate a #VALUE! error.*

##### The result of our two Conditional Formatting rules

When you apply the new rule, it will highlight all the cells that match the result of our formula. In this example I mentioned that there are some entries which have the same name, but the ID is different. This can be the case when two people with the same name register on a website or if two employees have the same name. They will have different IDs but the same name.

In column B the highlighting is made using the `SUMPRODUCT`

formula, while cells from column E are highlighted using the built-in Excel conditional formatting. In case of “Michelle Collins” the file contains 3 entries. Two of them are duplicates (rows 7 and 27), but there is another entry (row 12) which is unique. Highlighting the cells based solely on the name would result in a false highlight in this case.

This formula can provide an easy way to find duplicate rows as well. Just add more conditions to the SUMPRODUCT formula and it will reveal all your duplicates.

If you have additional questions or need help with your project, please let me know by posting a comment.

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