Arrange serial number for duplicate records

Hi,

I have data in Column A and will be adding more data in column A.

I want serial number in next Column (B) where serial number should be same for duplicate records.

For example in attachment

Cell A8 and A9 are same hence serial numbers also same - 7
Cell A13 and A14 are same hence serial numbers also same - 11
Cell A21, A22, A23 and A24 are same hence serial numbers also same – 17

Please help