The vlook function can easily help you in determining if a value that is in one column is in another.
This can be useful if the list is very long.
You should have data in the column, and have a duplicate in another.
Note: You can choose to have another kind of data in the columns.
Click on an empty column (1), and type in =IF(ISERROR(VLOOKUP(B2,$D$2:$D$16,1,False)),False,True) (2), and then click Enter on the keyboard.
Note: This is to know if the value in B2 exist in another column.
Further reading: How to create many to many lookup? How to Exchange Formula to Value?