One way to do it without macros would be via pivot tables.
To do that, highlight the identifier columns in each of the sheets - hold
control while moving between sheets and clicking on the various columns to
allow non-contiguous data selection.
Once all columns are highlighted (and this assumes no header row, or if
there is a header row has the same heading), use the Data, Pivot Table option
to create a new pivot table.
Select the data column for both the "Row" section and also the "Data"
section, setting the data section to count, descending (double-click on it -
count is default, and set options to sort descending by count).
This will result in a pivot table with the identifiers that appear most
frequently at the top (with the resulting number of times that they appear).
Therefore, if any appear more than once, they will be listed at the top.
Hope that helps.
