In a case reflecting problems that can occur when huge law firms represent huge clients, a Fulton County, Ga., judge has ruled that legal giant Duane Morris violated conflict-of-interest rules by representing subsidiaries of McKesson Corp. in Pennsylvania and then working for a couple against an unrelated McKesson subsidiary in Atlanta.

Judge Thelma Wyatt Cummings Moore of Fulton County Superior Court on Wednesday disqualified Duane Morris from the Georgia proceedings. She wrote that she was not persuaded by the firm’s efforts to “explain away the apparent conflict of interest” by relying on an engagement letter between Duane Morris and two other McKesson subsidiaries in the Pennsylvania case.