Both Cazimi and Retrograde are given the same dignity/debility score as each other, however keep in mind these are accidental dignities and not as powerful as essential dignities. Since they are of equal stength they will cancel each other out. Look to the essential dignity in this case.
I should also add that these positions have philosophical meanings to them as well which are both taken into consideration excluding the dignity count. For Cazimi shows us an individual who is protected by someone powerful. They are in the heart of the Sun, and with the Sun ruling the most powerful individuals in a society, Mercury (or any Planet) being found here is protected. Mecury retograde also philosophically shows the withholding and reversing of Mercury's signfiications (and what he stands for in a chart). It could take longer to develop (two step forward one steps back idea). Just because the dignities cancel each othe out, doesnt mean the philosophical meaning has gone.