Are there exceptions to the general rule that an Army Officer should not face administrative separation / board of inquiry for the same allegation twice?

Yes. AR 600-8-24, paragraph 4-4 c explains that if any of the below exist, an Army Officer can be made to face administrative separation / board of inquiry for the same allegation twice:

(1) Substantial new evidence that has been discovered that was not known at the time of the original proceedings despite the exercise of due diligence and that would probably produce a result significantly less favorable for the officer at a new hearing.

(2) Subsequent conduct that warrants consideration of discharge.

(3) A determination that administrative separation should be effected because of the unusual circumstances of the case.

However, facing administrative separation / board of inquiry for the same allegation twice is not automatic. There are several notice requirements that must be met.