With columns A & B, in column C a formula to pull down that will return an "m" for the minimum value of the A column "groups".

So for the three 123's the min is 4, so an "m" in first row of column C.
The three 125's min is 1 so the third row down in C would have an "m".
The one 126 is a non-dup so it too would return an "m" in column C

123 4
124 2
125 1
123 5
124 5
125 3
123 6
124 8
125 7
126 4

Thanks,
Howard