Personally I don't think the "chromosomes faltered" explanation will work
here. 'Gertrude' is indeed a diploid - a cultivar of I. pallida, but
judging from your photo, the odd-ball doesn't look much like I. pallida at all,
and if it is just the same plant with double the chromosomes it should
in theory, even if tetraploid, look rather similar to the main plant.
Of course it is very hard to tell much from the mini-photo in your post;
it's hard to make out much detail. Do you have more photos?
If it really is some sort of a polyploid sport, it would be very fun
I would suspect that a volunteer seedling would be more likely.