How to compare the values ​​of two columns for uniqueness in Google Sheets?

Question

Google

Column A contains 70 cells with values ​​(each word or phrase).
Column B contains 100 cells with values ​​(each word or phrase).
You need to compare the value of cell A1 with all the values ​​of the cells of column B and write a
"double" in C1 if it is repeated or the value of A1 if it is unique.
Then move on to comparison A2, etc.
Result: in column C, only the unique cell values ​​of column A and words like "duplicate" will appear, which are then sorted and deleted.

Answer 1

It seems to me that the article on Habré habrahabr.ru/post/198898 will be useful to you.

Answer 2

MATCH/Match