Within the original, Frank Herbert Dune series, Duncan Idaho seems to embody the second of two continuity errors (see my writeup for the tleilaxu). Within the space of one page (in book IV, God Emperor of Dune, Herbert seems to demonstrate an inconsistency.

"I died defending Paul and his mother in a cave-sietch beneath the sands of Dune. I have been returned to that planet, but Dune is no more. Now it is only Arrakis."
  -Chapter 7 (P45 in the ACE BOOKS paperback)

Duncan heavily implies that this is his last memory - the memory of his death. This conclusion is also supported by the conversations between him and the Bene Gesserit in book V, where they discuss the oddity of mixing cells from multiple lifetimes.

However, on the very next page, we are told that:

"Idaho remembered a strange child - twins, really..."
  -Chapter 7 (P46 in the ACE BOOKS paperback)

These twins (Leto II and Ghanima were not born until the end of Book II, and although there was a Duncan Idaho who witnessed their lives, he was the ghola Hayte. It seems illogical that the Tleilax would mix the original cells with the primary ghola.

Of course, it is possible that the many Idahos ordered by the God Emperor were gholas of the primary ghola Hayte, but if this is the case, why did Leto desire this, and why was such a fuss in the Vth and VIth books made of Idaho's 'multiple memories'?