We can set data validation for a certain list to automatically detect duplicate entries in Excel. Please do as follows:

1. Select the list you will prevent duplicate entries, and click Data > Data Validation to open the Data Validation dialog box.

2. In the opening Data Validation dialog box, select Custom from the Allow drop down list, and type the formula =COUNTIF($A:$A, A1)=1 into the Formula box. See screenshot:

Note: In above formula, $A:$A is the selected list/column in step 1 you need to detect duplicate automatically, and A1 is the first cell in this list/column.

3. Click the OK button to apply this data validation.

From now on, a warning dialog box as below screenshot will pop out automatically when duplicate entry occurs in the specified list/column.

If you have Kutools for Excel installed, you can prevent duplicate entries in a list or entire column with just one click.

Select a list or entire column you need to prevent duplicate entries, and click Kutools > Prevent Duplicate. See screenshot:

After setting, the below warning dialog box will come out automatically when detecting duplicate entries in the selected list or column.

The Prevent Duplicate utility of Kutools for Excel can help Excel users quickly apply the data validation of duplications prevention with only one click.

Demo: Automatically detect duplicates when entering values in Excel

Sometimes, you may need to detect or find duplicate entries in an existing list. Kutools for Excel’s Select Duplicate & Unique Cells utility can also help you solve it easily.

1. Select the list/column you will detect duplicates, and click Kutools > Select > Select Duplicate & Unique Cells. See screenshot:

2. In the opening Select Duplicate & Unique Cells dialog box, please check Duplicates (Except 1st one) option or All duplicates (Including 1st one) option as you need, and click the Ok button. See screenshot:

3. A dialog box will pop up and show you how many duplicate cells have been selected. Just click the OK button to close it.

So far, we have selected duplicate cells in the specified list or column as below screenshot shown:

Note: This Select Duplicate & Unique Cells utility can also highlight or color found duplicate cells if you have checked the Fill backcolor option. See screenshots:

The Select Duplicate & Unique Cells utility of Kutools for Excel is designed to find out all duplicates for selected column/list, and highlight all found duplicate cells or rows as well.

Demo: Detect /find duplicates in an existing list in Excel