Here's how to prevent duplicate entries using data validation in Excel:

1. Select the range: Choose the column or range of cells where you want to prevent duplicates. You can click and drag to highlight the cells or use keyboard shortcuts like Ctrl+Shift+End to select the entire data area.

2. Data Validation: Navigate to the "Data" tab on the Excel ribbon. In the "Data Tools" group, click "Data Validation."

3. Settings: In the "Settings" tab of the Data Validation window, under "Allow," choose "Custom" from the dropdown menu. This option allows you to define a custom formula for validation.

4. Formula for uniqueness: In the "Formula" box, enter the following formula: `=COUNTIF($A$2:$A$20,A2)=1` (replace the cell ranges with your actual data range).

- Breakdown:
- `COUNTIF` function counts the number of times a specific value appears in a range.
- `$A$2:$A$20` is the absolute reference to your data range (adjust cell references as needed). Absolute references ensure the range stays fixed even if you copy the formula to other cells.
- `A2` refers to the cell where the data is being entered (relative reference). As you enter data in different cells, the cell reference will change accordingly.
- `=1` checks if the count is equal to 1. This means the value being entered only exists once in the specified range.

5. Error Alert (Optional): In the "Error Alert" tab, you can customize the message that appears if someone tries to enter a duplicate value. This helps users understand the issue.

6. Click OK: Click "OK" to close the Data Validation window.

Explanation:

This formula essentially checks if the value being entered in the current cell (A2 in this example) already exists anywhere within the defined range ($A$2:$A$20). If the `COUNTIF` function returns 1 (meaning the value exists only once), the data entry is allowed. If it returns a higher number (meaning a duplicate is found), Excel will display the error message you set in the "Error Alert" tab, preventing the duplicate entry.

Additional Notes:

* You can copy the formula down to other cells within the selected range to apply the data validation rule to the entire area.
* Remember to adjust the cell references in the formula to match your specific data range.