One instance of 3 reps and one instance of 4 reps
An example of one instance of 3 reps is AAAABC. There are (6 choose 2) possible positions of the two non-repeated stimuli in a sequence of length 6. These are
12 13 14 15 16 23 24 25 26 34 35 36 45 46 56
There are 92 possible stimuli and the two stimuli must be distinct from each other and the repeated stimulus. This gives 92x91x15x90 combinations and a probability, dividing by $$926 $$ , of 0.00001.
For one instance of 4 reps e.g. ABBBBB there are 6 possible positions for the non-repeated stimulus (each of the 6 positions in the sequence). There are 92 possible stimuli and the repeated and non-repeated stimuli must be different giving 92x6x91 possible combinations and a probability, dividing by $$926 $$ , of 0.00000008. There can be no more than one instance of 3, 4 or 5 reps so the above scenarios cover all the possibilities for 3 and 4 reps respectively since there is only a single instance possible of 3, 4 or 5 reps.