Finding duplicate text in excel cells in real time and notifying you immediately when a duplicate appears?

Question

excel

Help make search for excel cells containing the same text and immediate notification of already existing cell with such information.
Example:
Let's say in column "A", in each next line we begin to write down the values ​​​​(let there be surnames).

1. A1 - Ivanov
   
2. A2 - Petrov
   
3. a3 - Sidorov
   

And as soon as we enter in cell A4 a value that is already in column A (for example, Petrov), a warning is immediately issued that such text is already present in the column.
Show the direction in which to proceed. Thanks

Answer 1

Of course, you can use a macro, but if there are 10,000 lines or changes are often made, it is not practical.
There are already built-in methods: