Sorry Mattrom, our posts crossed.
Yes, the first P is included in the wordplay but I took the "incidences" in the preamble to be the letter itself so, as in this instance, there are 2 P's in the same row or, to put it another way, 2 incidences of the letter P in the same row which, from my reading anyway, is contrary to what's said in the preamble ... "These letters, which must be highlighted in the completed grid, are positioned such that there is only one such incidence in each row and column"