If I have two or three tables with data and some of these data overlap between each table, how do I sum up the data without double-counting the overlapping data? And is there a visual way for excel to colorcode which ones overlap?

Example: (these are made-up data and a situation)

Table 1: (A1 to C3)
Office Paper Recycling Company 1
4/1/2010 5/1/2010 6/2010
100 150 69

Table 2: (A5 to C7)
Office Paper Recycling Company 2
4/1/2010 7/1/2010 8/2010
100 75 69

Table 3: (A9 to C11)
Compost Waste
4/3/2010 5/1/2010
50 69

The total amount of waste generated would be 100+150+69+75+69+50+69. (Going from top to bottom, left to right) in this case.
Only overlapping data would be the April 1st's 100 since both companies counted the paper waste to be theirs.
I want the Excel to recognize that the April 1st's 100 data overlaps and only add it once in calculating the total amount of waste.

Thank you for your time and help.