Return a "one" for the first instance of duplicate values and a "zero" for the other instances

I'm trying to write a formula to check for duplicates in one column.

It should return a 1 for the first instance of the duplicates and a 0 for the other instances. If the value doesn't have a duplicate it should return a 1 as well.

I tried to use

=IF(COUNTIF($B:$B, B6)>1,1,0)

but it returns a 1 for the other instances of the duplicates.

Any ideas?