I'm a little bit confused by your statement that the ipsos is both a correct quotation but can also be omitted. Is there some translation issue wherein the "ipsos" might or might not be part of the original poem?
>>IggleS+(OP)
As I understand, it is part of the original Juvenal quotation, but its omission does not substantially change the meaning of the sentnce.
>>IggleS+(OP)
"Quis custodiet custodes" means "Who will watch (the) watchers?" With the "ipsos" - "Quis custodiet ipsos custodes", it means "Who will watch the watchers themselves?" - i.e. it puts emphasis on "the watchers". Basic meaning for both is the same, but I think the original version used "ipsos"