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Assume your numbers are in column A, from row 2 down to row 1044
In any available column, put this formula on row 2:
=CountIf($A$2:$A$1044,A2)
and fill the formula down to row 1044.

It will show a value of 1 where the number in column A is unique to the
list, it will show a value greater than 1 where multiple occurrences of the
number on that row appear in the list.