Excel 2007 : Calculations using IF statement?

Good day everyone. I'm trying to sum cells if 2 other cells are equal which is easy enough but I don't want to display the value of the 2nd equal cell, which I am doing by setting that to zero. But if I have a cell that is not equal to another cell, I do want to display that value.

Hopefully, the attached spreadsheet will show what I'm attemtping. The 2nd block is how I would like it to come out.

I want to only show one value for the cells that have equal values in cell A but not list the value of the 2nd or subsequent matching cells. But if I have an occurence in cell A that is only listed once I want to show that value in cell C.

My formula is: =IF(A1=A2,B1+B2,"0") and I realize that's why a unique occurence is being listed as zero also.

Hope that makes sense. Please let me know if you need more info.

Thanks
Jay