A tautology is a statement that says the same thing twice (often as a result of poor word choice), or which logically is formulated in such a way that it is necessarily true. Examples of the first type of tautology include legal phrases such as cease and desist or last will and testament. Examples of the latter type might include If you gotta go, you gotta go, or Either it's going to rain, or it isn't.
Despite their complexity, tautologies don't convey any useful information.1 They do have the benefit of being true no matter what, however. So, on either count, it's not surprising to see KJV-onlyists using them.
You can keep whatever standard you are using to grade bibles, I find the KJV is perfect according to the standard I go by, which is the KJV. . . .
I find the KJV is perfect, however, when compared to the standard of perfection, the KJV. . . .
But, like I said, I find the KJV perfect according to the standard, which is the KJV. . . .
Well, there you go. When you compare the KJV to itself, amazingly there are no differences between them! Wow! It's magic!
Problem is, it really proves nothing, because it proves too much. Why should this proof be limited to the KJV? Set any other Bible translation up as the "standard," and compare it to itself. Not surprisingly, it turns out to be perfect!
In fact, by the very same logic, I can declare any arbitrary book to be the "standard": let's say It, by Stephen King. When I compare It with It, I find to my great astonishment that they agree perfectly in every detail! Astounding! By marke's reasoning, this must be evidence of divine inspiration. There is no God but Pennywise the Clown, and Stephen King is Its prophet!
It surprises me, incidentally, that to date I haven't blogged the most common of all KJV-only fallacies: petitio principii, better known as "begging the question" or "circular reasoning." You certainly can't get any more circular than marke's outright declaration that when he assumes the perfection of the KJV from the outset, he inevitably discovers it to be perfect.
1 Tautologies may, however, be used figuratively conveying non-propositional truth. For example, If you gotta go, you gotta go simply expresses resignation to the inevitability of a bathroom break. And the infamous quadruple tautology in Unam Sanctum, the papal bull of Pope Boniface III—"We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff"—is there for emphasis.