A fast way of flagging common items (duplicates, as you call it) between
Sheet2 and Sheet1 would be:

=--ISNUMBER(MATCH(A1,Sheet1!A:A,0))

1 means duplicate, 0 not.

genoq wrote:
I have a workbook with 2 spreadsheets, each with containing a list of part
numbers. On the second spreadsheet, I want to compare lists and identify
duplicates from the first list, without changing the sorted order of either
list. What is the easiest way to flag these duplicates on sheet 2