Oh mighty Excel Sorcerers and Corvette lovers everywhere, wons't thou please favor us with a wicked cool "=if" formula that I used to know how to write but have forgotten?

I have a column of values. They're all numbers currently, but I used to use this formula for words too. I will sort the column so that any duplicate values will be listed sequentially, like this:

1
2
3
3
3
3
4
5
6

I'm looking for this:

If A2 = A1, write "duplicate" in B1. If A2 does not = A1, write "not duplicate".

Then I would paste the formula down column B to get this desired result:

1 not duplicate
2 not duplicate
3 duplicate
3 duplicate
3 duplicate
3 not duplicate
4 not duplicate
5 not duplicate
6 not duplicate

My everlasting gratitude awaits your formulaic wizardry.