Column A has numbers in it and some of these match.
Column B has numbers in it and some of these match.
The columns are already sorted by A then B.

What I need to do is where a number matches in column A, I need a number €˜1
inserting in column C next to the highest number in column B. This needs to
be copied down the sheet.

A B C
1 1 5 1
2 1 2
3 1 1
4 2 3
5 3 2 1
6 3 2
7 3 2
8 4 3 1
9 4 2
10 4 2
11 4 2
12 4 1

As you can see in the example above if there is more than one number the
same in column B then only one €˜1 needs to be inserted.

If there is only one unique number in column A then nothing needs to be
inserted in column C.

Thanks in advance for any help.