Conditional Sum?

I have a dataset of annual values which I need to combine into an array. Each year's data is in a separate worksheet, and contains between 4000 and 6000 rows. I set up a master sheet listing all unique locations for the entire period of record, finally got over the tics and brain damage from that one, and put in a VLOOKUP function for each column to retreive that year's data (examples-
=VLOOKUP($A2,'1995_5580'!$A$2:$E$5581,5,FALSE) =VLOOKUP($A2,'2002_6052'!$A$2:$E$6053,5,FALSE)). Works great. I still need to get something other than #N/A for not found values, but I can figure that out. Checking the results, I discovered that a very small percentage of the locations have more than one entry. In order to make this matrix useful, I need to have a sum of all values that match a given location. VLOOKUP only gives me the first match.

Now, the problems- I can't post a workbook, because it's confidential data. I would really prefer to stick with functions and not macros, because a lot of macros seem to get picked off by the firewall (I'm working on a network drive). I've seen similar problems addressed here with PivotTable (won't work here because a large number of locations are missing in one or more years and each year has a different number of data rows) and array formulas (data is on 23 separate worksheets). I really, really don't want to go through 100K+ rows of data to manually find duplicates and enter them into the master sheet. Not only would it short out my few remaining brain cells, it's inelegant.