Another way to play it ..

Assuming source data is running in A2 down

Place in B2:
=IF(A2="","",IF(COUNTIF($A$2:A2,A2)1,"dup",""))
Copy B2 down as far as required

Col B will flag duplicates in col A as "dup".
We could then easily autofilter* on col B for closer inspection of the "dup".
*Data Filter Autofilter
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Max
Singapore
http://savefile.com/projects/236895
xdemechanik
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"acesandspades101" wrote:
I have created a worksheet that lists identification numbers for valves and
instruments on filtration machines. Now these identification numbers are
unique because they are a combination of both letters, number and characters.
I need to find a way to check if any of these indentifaciton numbers have
been duplicated in a column. No 2 numbers are the same and again I need to
know how I could check a column in excel for possible duplicates to correct
them.

Thanks