Excel 03: Flag where same item arises in separate columns.?

I have a list of addresses with postcodes (EG. SL6 2YJ). I receive a further list. Both are in columns in Excel 03. Using the post code to spot duplication, I want to place a mark beside the entry/cell in column 1 if the same value appears anywhere in column 2.
I imagine a formula like "IF value in col 1 cell 1 = any value in col 2, place a mark, otherwise leave blank". Have tried to create this but not working for me. Thank you for any advice.

You will need an array formula...

Let the existing postcodes be in F2:F202
Let the new arrivals be in Y2:Y11
Then in Z2 put:
=Sum(if($F2:$F202=Y2,1,0))
don't press Enter...
Hold Ctrl and Shift and then press Enter.
It should look like this:
{=Sum(if($F2:$F202=Y2,1,0))}

Drag this formula down to Z11...

You could try the MATCH function which will search an array for the cell value and tell you where in the array the match is.

This is explained in detail in Excel Help menu - open it and enter MATCH in the search area.

I placed the orginal Post Codes in B1 to B3 and second set in F1 to F4. Then put the following formula in C1 to C3.

=IF(ISNA (IF ((B1=VLOOKUP (B1,$F$1:$F$4,1,FALSE)), "Y","")),"","Y")

I have had to put spaces in this formula so that it displays correctly in Answers. Remove these once you've copied it into your spreadsheet.

Copy this into the cells C2 and C3.

If there is a duplicate it will return Y next to the orginal post code. If it fails to find a duplicate the adjacent cell is left blank (as you proposed).