Formula for categorizing and labeling duplicate entry

Folks,

I was hoping someone could help me out. I will continue trying to solve it, but I think this problem is far beyond my Excel ability, which is poor.

I have hundreds of cells in Column A, some of which are duplicate cells. Now, I want Column B to indicate whether the entry in Column A is duplicate entry or not. If the entry is duplicate, I want Excel to indicate as much in the first such entry.

By way of example, this is what I'm hoping the end result will be:

Individual	Existing (Y/N)
John	No
Jane	No
John	Yes

Please let me know if you guys have any clue how to do this. I'd do it manually but that is a cop out, and I have a range of 1000s of cells.