Create A Validation List

Hi all,
I have a list of letters in a column ie. A2 = A, A3 = B, A4 = C and so on.
What I want to do is add all of these letters to a drop down validation list in a cell.
I can do this fairly easily but only by adding each row as a different value (i.e. the user will have to chose between A, B or C).
What I want is for the user to be able to chose any combination of the letters (i.e. AB, AC, BC, etc).
I have found the following example on the internet:

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However, this does not return only the unique combinations (i.e. it returns ABC and CBA).

Can anyone advise me on how to do this?

Once I have all of the possible combinations, my plan is to use them to create a string and add that to the validation list in the cell.

I hope all of this makes sense?

Thanks snb (AGAIN!)
It always take me a while to decipher your code but it always turn out to be the most logical and efficent way of achieving what I need.