Highlight cell if value is present in any cell in another column

Please select ColumnA, Format, Conditional formatting..., Custom formula is:

=match(A1,B:B,0)>0 

with formatting of choice.

Do conditional formatting on each cell with the formula:

=EQ(VLOOKUP(A1, B:B, 1, FALSE), A1)

And format the cell to a different color if it matches.

I shared an example here (this link will ask you to make a copy in your own Google Drive account):

https://docs.google.com/spreadsheets/d/1IovLko1cF2guKnIalCyE0uSbCvMDYLgL0BZHt35znXI/copy

You would add conditional formatting to A1 and then copy to rest of the cells in the column. Set the conditional formatting to custom formula and make the formula:

=COUNTIF($B$1:$B$8,A1)

And set your color. It will highlight the cells in the A column if they exist in the B column. (Keep in mind I Am assuming your range is B1:B8)