Give warning when duplicate values are placed in different keywords - excel

Question

Let's say I have a sheet with multiple columns but to make the example I only put 2.
What I want to achieve is to give the user a warning when the same person get's placed in different teams. (It's possible to have duplicate persons, but it's not allowed to have them in seperate teams)
First I thought I'll filter it so I can only see the duplicate and then check if the same person get's placed in 2 different teams. But now I see it's not possible to filter the duplicates. Then I thought using conditional formatting and check if the cell.color.interior is changed but I noticed it does not change it! Solutions provided on stackoverflow does not suffice for me. Neither do I want to a pivottable or an extra column since my sheet is already overcrowded.
Example:
Value A Value B
Tom Team 1
Ben Team 1
Tom Team 1 <- possible
Elle Team 2
Tom Team 2 <- not possible, give warning!
Rick Team 2
And the list goes on.
Does someone know I can give the user a warning when placing the same person in different teams?
Or how to get to see the duplicate values in the sheet or get it in a range in vba?
Thanks!

Answer 1

Okay then, since you cant sort the table it requires a rather more brutal or complicated approach.
1) First a simple, quick approach
Although you could still add a temporary column, number it, sort the sheet based on the Value A field, then just iterate through the cells, looking for instances where
Cells(curRow, rowNumOfValueA).value = Cells(curRow-1, rowNumOfValueA).value
When this happens, check that the value in column 'ValueB' is the same for each item. If so, continue onto the next row. If not, mark the row in some way - setting the interior colour is often a good way to do it. You can then sort the table according to the added column, delete the column and then take action.
2) Second a brute-force
Starting at the first row, get the value in ValueA and ValueB, call them for example findMeA and findMeB
a) starting at the following row, check to see if ValueA matches findMeA. If it doesn't, move on to the next row. If it does match, ensure that ValueB matches findMeB too. if not, mark the row or add its index to an array for reporting when the sanity-check is done.
b) move to the next row, if it's empty, return to (2). If it's not, return to (a)
As you can probably see, the brute force approach gets exceedigly nasty cpu-wise as the number of rows goes up. With just 10 rows of data, it's 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 searches that need to be done, unless you make a mistake of logic and check every row against every row. Just 10 rows would be 100 checks. - 10^2
If, on the other hand, you sort the table first - you can get away with just N checks for every N rows. 10 rows takes 10 checks, 1000 rows takes 1000 checks. (I can't be bothered to find out how to easily calculate 1000 + 999 + 998 + .... + 1, but it's huge. Actually, it's about 500,000 (1000 + 999+1 + 998+2 + 997+3 ... 499+501 + 500). - that is 1000 + 499*1000 + 500
So, the brute force takes exponentially longer as the list grows, but the sort-first method takes linear time. Twice as big equals twice as long.
Sorted method: 10 items - 10 checks. 1000 items 1000 checks.
Brute Force: 10 items - 55 checks. 1000 items 500,500 checks.
I.e at worst case, brute force takes 500 times as long as a sorted approach. If it was only a few items, I'd just suggest brute-forcing it. Over half a million checks with VBA doesn't seem so pleasant though, so I'd sort the sheet, or at the very least - make a copy and sort then operate on that, before reporting the problems with the actual sheet.