Page "Res judicata" Paragraph 18

Regarding designation of the parties involved, a person may be involved in an action while filling a given office ( e. g. as the agent of another ), and may subsequently initiate the same action in a differing capacity ( e. g. as his own agent ).

In that case res judicata would not be available as a defense unless the defendant could show that the differing designations were not legitimate and sufficient.