True/False Duplicate Formula

I have a list of numbers in column A. In column be i need a true/false formula to tell me if the number listed in column a is a duplicate.

Would you only want the 2nd+ occuarance of the duplicate to be true?
Or should the first occurance of the duplicate also be true?

If the first occurance of the duplicate should be FALSE, try this in B1 and filled down.

=COUNTIF(A$1:A1,A1)>1

in B1 and copy down

=IF(COUNTIF(A:A,A1)>1,TRUE,FALSE)