How to check for the presence of a value (Google Spreadsheet)?

Question

Google Sheets

Let's say there is a column "1" with certain values.
How can I make sure that when I try to enter a value in column "2" that is already in column "1", this cell blocks the input?
Or just painted over in any color (for example, red).
ps I'm using a google spreadsheet, not an ixel.
If you don’t know how to do this in a Google spreadsheet, describe the implementation scheme in ixel.

Answer 1

Using the MATCH function ( https://productforums.google.com/forum/#!msg/docs/... the idea is there).