As an Army Officer, can I face administrative separation / board of inquiry for the same allegation twice?

Maybe. Pursuant to AR 600-8-24, paragraph 4-4 b, with some limited exceptions, “no officer will be considered for elimination for reasons … because of conduct that has been the subject of administrative elimination proceedings that resulted in final determination that the officer should be retained in the Service.” In other words, in most cases an Army Officer that has already gone to a board should not have to go to another board for the same alleged underlying misconduct. This is true even if the board had been initiated for the underlying alleged misconduct and HRC later finds a referred officer evaluation review (OER) that references the derogatory information.