How to prevent the same text from being entered in Google Doc?

Question

Google Sheets

There is a table in Google doc, for cold calling, the question is how to make it so, for example:
I dialed a client, entered his number into the database, then I accidentally stumble upon his number again, how to make it so that duplicate numbers cannot be entered , or make it the same filling as in the screenshot.

I hope you understand my question.

Answer 1

Try Data Validation (Data Next > Data Validation)
Basic Formula =COUNTIF($A:$A;"="&A1) < 2
Setting Example

Message if Duplicate Data Entered

Table Example How do I prevent the same text from being entered in Google Doc? ...

Answer 2

It is better to keep all clients in CRM.