Describe FAQ/combinatorics/inst2rep1 here.
Derivation of 1 instance of 2 replicates of one of 92 stimuli in a sequence of length 6
An example of 1 instance of 2 replicates is AAABCD ie three repetitions of the same stimulus in the sequence of length 6. There are (6 choose 3) = 20 ways of ordering the repeated stimulus. These are
123 124 125 126 134 135 136 145 146 156 234 235 236 245 246 256 345 346 356 456
There are 92 stimuli that can be repeated and there would be three distinct simuli not equal to the repeated stimulus in the remaining three positions giving 92 x 20 x 91 x 90 x 89 possible combinations of a single incidence of 2 repeats of a stimulus giving a probability when divided by 92^6 of 0.002.