Duplicate check in Excel?

Question

Macros

Good day!
The Excel sheet has a column with numbers, as a rule, when a value is added to a cell, a check for duplicates does not occur. How to set up to check and the appearance, for example, of the message "This entry exists" in case of a duplicate?

Answer 1

Select the range of cells and click the Data Validation button on the Data tab.
In the example, this is the range A1:A10.
Enter the formula
=СЧЁТЕСЛИ($A$1:$A$10;A1)<=1

Screenshot 1

On the "Error Message" tab, write the error message:

Screenshot 2