^haha slippery slope...
from what i've seen, the level of effect received during consecutive trips varies from person to person; though it seems that the effect is lessened to a degree across the board.
myself, back in my day of very regular psychedelic experiences i would always be satisfied from the second day of tripping, even if the same dose was taken 2 days in a row. however, i could definitely notice a lessening of the intensity.
the way i remember describing it to people years ago, was that when you trip regularly and become used to the psychedelics mind state, it seems that your conscious self quickly gains the ability to coordinate 'itself' within the mindset. the effect of this is, from my experience is the consequent 'bringing of the self into the experience', which in effect holds back the ego annihilation part of the experience....lol, does that make any sense, it's rather hard to put into words. largely the difference of the feeling is a sense of groundedness, or 'not being blown awayness' during the experience.....
now all of that being said, i think the difference in one's experience of multiple days in a row of tripping is partially determined by the ego's own limitations..whereas normally tripping this is the last thing holding you back. myself, practicing much meditation and consciousness exploration without the use of psychedelics;i was always still quite satisfied with the experiences. i just noticed it felt less alien, or weird....almost begining to seem 'normal' to be tripping, though i would no doubt be tripping still....
the ultimate deterent of such practices from my experience is the crossing over effect...lol, quite intimidating...which is: When you start feeling 'normal' while tripping, it's easy to feel 'trippy' while sober. some may feel that is a desirable effect, to each their own...HPPD or HPPO (hallucinogenic perceptual order: order brought about through the persistent perceptual after-effects of hallucinogenics) :D i used to argue i had the latter....
i don't trip so much anymore, a few times a year and generally in a more ceremonial context...certainly with absolute intentions and respect, my years of nonchalant psychedelic adventuring are past. and so it rarely comes up to consider a second day of tripping after a first...however i don't disclude the possibility.
i've had very spiritually transformative experiences eating psychedelics multiple days in a row, so i regret none of what i've gone through, and i'm open the possibility of a weeklong perceptual workout lol....but moderation, disciplined self-preparation, and solid intention are the right ingredients for positive and evolutionarily-beneficial experiences.... :D
sorry for the lengthiness, didn't mean to make it too personal, but i feel the subjective nature of one's own recollective experience is the best stone to throw when it comes to moving towards objective truth.