Well, I have explained my view on this very recently but I forgot where. I'll repeat myself:
If you redose something like acid (I think this goes for many other compounds as well especially psychedelics) you extend the phase of the trip you are currently in and all the following phases. You will not start all over again.
I don't know if you ever noticed this but trips are often like a story being told with different chapters. You can divide them in onset, comeup, peak, plateau, comedown and afterglow.
Often these phases are not only a measure of intensity of effects but also of content and quality. The comeup and first part of plateau are often full of conflicting effects, much contemplation and thinking over, analyzing etc. In conclusion of this you often get a sort of resolution and clarity. You still feel the trip but there are then a lot less thoughts flying over your head and answers rule over questions. Sometimes you can get clarity very early in the trip.
You can re-drop acid very late but it will keep you in the afterglow like clear phase (I love this by the way), the earlier you redose the more stretched out all of the different phases will be in my experience. This also depends on the size of the dose making it all the more complex.
But also remember that redosing late means that you have to compensate for tolerance. Tolerance builds up quickly, if you redose at about T+7 or T+8 you would already have to almost double your added dose. It's tricky though because often this is definitely necessary while at some times it can increase the intensity of effects very dramatically and a little unexpected.
This question is a very common one, actually it is so common that I am starting to believe it deserves its own separate LSD subthread. I will check this out later but expect this thread to be assimilated by this merger.