Summing across multiple sheets by specific name, VLOOKUP?

After tinkering most of last evening, I've successfully implemented the second half of what I was trying to do...

"Then, after I have a list of all teams that participated throughout the month in column A
...I want 'Cumulative' sheet, Column B to look at the corresponding row name in column A...
... find that name in 'Week 1' sheet, and...
... copy the corresponding column I value from the row in the Week 1 sheet in which that name appears to 'Cumulative' sheet column B.

Then repeat for column C in Cumulative sheet, for Week 2. Then column D for Week 3, etc...
I haven't tinkered with macros before, so that would be an adventure for me. But if it's the only way, I'll have to learn how to I suppose. Thanks again."

Some variation of... =IF(ISERROR(VLOOKUP(A2,Week1!$A2:$I26,9,FALSE)),"-",VLOOKUP(A2,Week1!$A1:$I25,9,FALSE))
...for each cell in the 'Cumulative Sheet'. So as it stands now, if I type in any of the team names from column A of the previous sheets, the cells in 'Cumulative' sheet automatically fill in the corresponding scores.

The only other thing I'd like to implement, is the first part... "Ideally, I'd like column A in 'Cumulative' sheet to look at previous column A's in Weeks 1 through 5, and add the name to this column if it doesn't already appear."

I think I've figured out how to do it if I was only looking at an array from a single sheet, using something similar to =INDEX($A$2:$A$20, MATCH(0, COUNTIF($B$1:B1, $A$2:$A$20), 0))

I'm having difficulty figuring out how to look at A2:A26 from sheets Week1 through Week5, to perform the INDEX function and only return unique entries. Any suggestions on that one?