I would like to automatically number the occurrences found in column A in
column B. For example:

A1: One B1: 1 (First occurrence of One)
A2: One B2: 2 (Second occurrence of One)
A3: One B3: 3 (Third occurrence of One)
A4: Two B4: 1 (First occurrence of Two)
A5: Two B5: 2 (Second occurrence of Two)

I am partially there using:
B1 =IF(A1<"",COUNTA($A$1:A1)&".","")
B2 =IF(A2<"",COUNTA($A$1:A2)&".","")
B3 =IF(A3<"",COUNTA($A$1:A3)&".","")
B4 =IF(A4<"",COUNTA($A$1:A4)&".","")
B5 =IF(A5<"",COUNTA($A$1:A5)&".","")
but, I need to somehow add the conditional if A changes B restarts numbering
at 1. The file I'm going to copy into column A contains about 3,000
lines/rows.

Thanks for your help...