I think this one is worth mentioning; granted, it doesn't "belong" to anyone on our forum, per se:
Student of mine came to me with an interesting theory he read on a Japanese board:
[…] the author suggests that Teach may have more than one DF within him and that one is a Zoan, possibly tiger. How is this possible? Since the power of the Yami Yami fruit absorbs even DF's power, is it possible that more than one can exist in his body via the power of that particular fruit?
The whole absorption thing definitely makes it plausible…
Shanks never suggested the scar was given to him by BB with the Yami Yami power. He could have eaten the Zoan fruit THEN the Yami Yami.
But isn't it the Yami Yami fruit that would have allowed him a second fruit?
They're suggesting that the nature of the fruit in being in his body, absorbs the power of the other. It doesn't matter which way you take it. If the moment you bite into a DF you gain it's power, that means that the moment he bit the Yami Yami fruit, it suppressed/absorbed/I-don't-know-what his other fruit's power. By the nature of having the Yami Yami fruit, he can absorb/hide the other power, then unleash it when needed.
Perhaps that is the word I should have chosen. Having eaten the Yami Yami fruit he may be able to neutralize the so-called 'curse' and store multiple powers within him.
One more time. The author of the theory suggested that in eating the fruit, with the fruits innate ability to neutralize DF powers it's possible that it may react the same way to OTHER PAST EATEN, PRESENT OR FUTURE DF's within a single person.