When will a court refuse testimony from a witness who offers to swear both an oath and a separate oath?

This case has an odd twist in that the defendant offered to take the traditional oath if he could also take his own. His own oath superimposed on the traditional one would have taken nothing away from the commitment to tell the truth under penalties of perjury and, indeed, in the defendant's mind imposed upon him a higher duty. Under these circumstances the district court clearly abused its discretion in refusing the oath and preventing the defendant's testimony. We do not have a case where the witness offers to swear only to a cleverly worded oath that creates loopholes for falsehood or attempts to create a safe harbor for perjury as in United States v. Fowler, 605 F.2d 181, 185 (5th Cir.1979), cert. denied, 445 U.S. 950, 100 S.Ct. 1599, 63 L.Ed.2d 785 (1980), where the court properly refused testimony from a defendant who would not say so much as "I state that I will tell the truth in my testimony," and was willing to say only "I am a truthful man" or "I would not tell a lie to stay out of jail."