Give this a try:
=IF(COUNTIF($A$2:$A$14736, $A2)<2,"",COUNTIF($A$2:$A$14736, $A2)-1 & "X")
In the above example we have an original value and two duplicates.
If there are no duplicates (only the original), then the formula leaves the cell blank.
(if you want to count the total number of items (original + duplicates), then just drop the -1 in the formula)
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