Gmail sheets and data integrity

Say, I have a Gmail sheet with name, surname and telephone number . I am trying to ensure that no duplicates of the telephone number are created. Apparently the users search before they enter each new entry, but sometimes they forget to do this and end up with duplicates. Is there a way to enforce data integrity as in a database by creating a primary key structure?
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dsackerContract ERP Admin/ConsultantCommented:
You will have to use an adjacent column for this.

For instance, in C1, put in the following formula:

Now, put a heading in B1, perhaps something like "My Value".

In B2, enter 1
In B3, enter 2
In B4, enter 3
In B5, enter 4
In B6, enter 1   (your first duplicate value)


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