Hi,

I have a list of lessons that occurred like such

Client Name | Date Occurred | etc.

Bill | 5/25/14
Bill | 5/31/14


Tom | 4/26/14
Bill | 6/12/14
Tom | 7/13/14

I need to determine for each row whether or not it is the last lesson in a new column called "last lesson?" which can just be yes or no. Right now I've been able to do it with:
=INDEX(A:B,SUMPRODUCT(MAX((A1:A5=A1)*(ROW(A1:A5))) ),B)
to give me the date of the last lesson, and then another function checking that date against the date occurred but this makes excel grind to a halt as this data set has 25k rows.
Is there an efficient way to do this?

Thanks!