"Anonymous. Which as nothing to do with should a judge , judge their own pupils in a competition. Whether there be one judge or eleven is irrelevant and should be disregarded by the jury."
On the contrary, it is quite relevant. The more judges, the less the importance of each opinion.
"The question remains is it fair for a judge to judge their own pupils."
As long as it is allowed by the rules, it is permimtted. I don't think that anyone believes it to be ideal - we'd all much prefer to have expert judges who don't have a history with the competitors.
But given the frequent practical need to choose between experts who coach some of the competitors in the event, and dancers who are not expert enough for any of the competitors to have any interest in taking lessons with, I'd much rather go with those that the competitors respect enough to study with.