Alan Beban wrote...
....
If your data is in B1:Bwhatever, then enter =1 in A1; then in A2 enter

=IF(B2=B1,A1,A1+1) and fill down

You're assuming all duplicate names are grouped together. If duplicate
names wouldn't necessarily be grouped together, use 1 in A1 and

A2:
=A1+ISERROR(MATCH(B2,B$1:B1,0))

and fill down.