Numbering rows with duplicate names

[Kristy K]

I have a list of 4000 names. I need to number the rows in Column A, but I
need any dupliate names to have the same number and then have the numbering
continue down the column.

Thanks for your help.

 

[Fred Smith]

How about something like:

b1: 1
b2: =if(a2=a1,b1,b1+1)
copy down