Prevent Duplicate Data Entry using Cell Validations

We all know that data validation is a very useful feature in Excel. You can use data validation to create a drop-down list in a cell and limit the values user can enter. But, do you know that you can use data validation in a multitude of ways to prevent users from entering wrong data?

Here is a practical application: Prevent users from entering duplicate values in a range of cells.

For eg. you are making an invoice. Wouldn’t it be cool if Excel prompted you when you enter a duplicate line item so that increase the item quantity instead of repeating it.

Here is a 3 step tutorial to do just that.

Step 1: Identify the range of cells where you want only unique values to be entered

This is simple. All you have to do is find the range where you want to control the user input. Lets say the range is B4:B11

Step 2: Set up Data Validation to prevent duplicate entries

Select the range (B4:B11) and go to data validation (Excel 2007: Data Ribbon > Data Validation, or press ALT+AVV)

Now, specify the validation type as “Custom”, this will allow us to use formulas to check for valid data. In our case, we need check if a particular entry is duplicated in the range B4:B11. This can be easily done using COUNTIF formula [learn COUNTIF Formula in plain English].

Go to the formula field and type the countif formula like this: =COUNTIF($B$4:$B$11,B4)<=1

Also, you can set up the “Error Alert” so that you can show a custom message when a duplicate value is typed, like “You have already added that product” message.

Once you set up error message, it will show up like this:

Step 3: Sit back and Relax

The third step is all too familiar. Now that you have prevented duplicate entries in your spreadsheets you can sit back and relax. May be read a few more spreadcheats.

More kickass stuff on data validation (and working with duplicates) using excel

There are a ton of useful articles on the site related to data validation and working with duplicates using excel. Here is a sample. Get started and learn something useful today.

• Data Validation Tricks in Excel [with downloadable tutorial]
• Remove Duplicate Values from a Range in Excel
• Using Array formulas to find if a list has duplicates
• Working with duplicate items in Excel [several examples]
• Examples & Articles on Excel Data Validation

This post is part of our spreadcheats series (yes, the series is still going after one year 😛 )