Yes, with the help of Data Validation in Excel you can create a validation for an alert of having duplicates...
Data validation is a tool that helps you control the kind of information that is entered in your worksheet. With data validation, you can:
--provide users with a list of choices
--restrict entries to a specific type or size
--create custom settings
You can use Data Validation to create a dropdown list of options in a cell. List items can be typed in a row or column on a worksheet, or typed directly into the Data Validation dialog box. Then follow the steps for creating validation..
1. In single row or column, type the entries you want to see in the drop-down list. (Note: The list must be in a single block of cells )
2. Name the List Range
3. Apply the Data Validation by Selecting the cells in which you want to apply data validation.Then from the Data menu, choose Validation. From the Allow drop-down list, choose List. In the Source box, type an equal sign and the list name. Now click OK
You can also Use a Delimited List:- Instead of referring to a list of items on the worksheet, you can type the list in the Source box, separated by commas.
If you want to allow users to type items that are not in the list., turn off the Error Alert.
Hope now you finally understand the way for validating cells.