Put the VLookup formula in the cell next to the first keyword in the other list.
=vlookup(click on first cell you want to check against the old list, highlight the columns you want to check it against in another sheet, type the number of the column you want to appear, type FALSE)
If your keywords are in column A and you put a 1 (or anything for that matter in column B, the the number for this: "type the number of the column you want to appear" will be 2.
Copy that formula all the way down to the end of your keyword list and the resulting cells with a 1 in them next to the keyword list will be your dups.