Squall mentioned to me in a PM that he won't be posting here for some time, but did suggest that I post his recently developed explanation for the similarity between Tidus and Shuyin. Here, quoted from the PM, is what he said.

Quote Originally Posted by Squall of SeeD
The largest misconception concerning Final Fantasy X and X-2 is that Tidus is the dream version of Shuyin. This is simply not the case and I will here explain why, and then offer an explanation for the striking resemblance between the two..

As is well known, Dream Zanarkand had existed for 1000 years. Dream Shuyin should have long since died, as the people of Dream Zanarkand are subject to the same mortality as people in the rest of Spira. Further still, it's assured that the Fayth did not wait 1000 years to project memories of Shuyin, as it's stated by the Bahamut Fayth that the memories of the Fayth on Gagazet were utilized to summon all the buildings in the city and even all the people who lived there:

Quote:
Fayth
"The remaining summoners and the townspeople that survived the war..."
"They all became fayth--fayth for the summoning."

Tidus
"The summoning... You mean Sin?"

Fayth
"No. I mean this place."
"A Zanarkand that never sleeps."

Tidus
"What?"

Fayth
"The dreams of the fayth summoned the memories of the city."
"They summoned all the buildings, all the people who lived there."



Dream Shuyin would have been summoned some 1000 years before Tidus' birth, along with the city itself. As such, Dream Shuyin would have also died some 1000 years before Tidus' birth.

Also notable is that even had a Dream Shuyin not been created 1000 years before, there's no reason for Yu Yevon to be making people at this point any longer in the first place, as the people of Dream Zanarkand are living, breathing, reproducing people. The population of Dream Zanarkand would take care of the matter of providing future generations on their own. For that matter, we know that Yu Yevon didn't randomly create Tidus, as we see the parents that he was born of. This is not to say, however, that Tidus couldn't possibly be a descendant of Dream Shuyin. It's not at all uncommon or unheard of for characters in Fantasy tales or RPGs to bear a striking resemblance to their ancestor (such as Marle in Chrono Trigger looking almost exactly like Queen Leene, her ancestor of five generations before).

Being aware of the matter of time passing the same in Dream Zanarkand as it does in Spira, some have come to believe that Dream Zanarkand must reset occasionally, as they see no other explanation for Tidus' resemblance to Shuyin other than Shuyin having been the basis for him. However, it's not at all accurate that Dream Zanarkand exists as a temporal causality loop that resets every few years. Most notably, this isn't suggested by the game, nor do we see evidence for it within the story. For that matter, there's not even any reason implied for such a thing to take place, nor any apparent purpose it could possibly serve.

Striking resemblances are no stranger to Fantasy tales, nor to Japanese tales in games or Anime (think of Taurus/Turlis and Gokou in Dragon Ball Z's third film, The Tree of Might), nor to even Final Fantasy games (Maria and Celes in Final Fantasy VI, for instance), and quite often there is no relation or previous connection of any kind between the characters bearing resemblances to one another.

With all that said, a common explanation for the similarities between the appearances of the two -- and it is certainly a striking resemblance at that -- is that Dream Shuyin may be Tidus' ancestor. While this is certainly a possibility, something to be kept in mind concerning the people of Dream Zanarkand is that their bodies aren't so much normal flesh and blood -- in which case genetics plays the core role in the appearance of a child -- as they are Pyreflies that have manifested into a living body, with the basis for their formation being in the memories of the Fayth. As the Bahamut Fayth said, the Fayth's dreams summoned "the memories of the city" of Zanarkand. As with any Fayth, the appearance of the Aeon produced is going to have a basis in the memories of the Fayth from which the memories come. The Aeon that comes as a result need not appear as an exact replica of that upon which it is based. Take the Fayth statues seen in the Temples for instance: They bear strong resemblances to the Aeons which they are used to produce, yet the appearances are not exact. An example of this is the Ixion Fayth:

Ixion's Fayth.
Ixion.

As can be seen here, the memories of the Fayth are merely used as a basis for the Aeon that the Summoner will manifest. Similarily, the basis of Dream Zanarkand's Blitzball stadium is the Yevon Dome seen in the ruins of the original Zanarkand.

With the fact in mind that a Fayth's memories need not be produced into an exact replica when summoned as an Aeon, consider again the Bahamut Fayth's words to Tidus:

Quote:
Fayth
"The dreams of the fayth summoned the memories of the city."
"They summoned all the buildings, all the people who lived there."



The dreams of the Fayth summoned their memories of the city. As they had known it. With that in mind, recall that Yu Yevon continues to tap into their dreams to access their memories and summon Dream Zanarkand and its citizens. While its citizens are the descendants of those who were originally summoned to live there, they are still summoned from the Fayth's memories of the city, even while time continues to advance there and these people are each different and unique from one another and their ancestors. Thus, rather than genetics playing a role in the appearances that children will take, the memories of the Fayth play this role. As such, all who are summoned into Dream Zanarkand have some basis in the Fayth's memories of the city.

With it being the Fayth's memories of Zanarkand that Yu Yevon uses to summon Dream Zanarkand and the Aeons that inhabit it, the appearance of everyone in the city is based on the Fayth's memories, and, further, there is a limited number of appearances that could be used as templates for the appearances of Dream Zanarkand's people. As such, variations are made on the templates, possibly to prevent more than one person from bearing a certain exact appearance.

Most likely, this is the reason for which Tidus bears an appearance similar to Shuyin, as well as a similar fighting style (as seen in the battle with Shuyin at the end of Final Fantasy X-2): With a limited number of physical templates from which to make Aeons in Dream Zanarkand due to the appearances being based on the Fayth's memories of Zanarkand -- which we know they must, as the Bahamut Fayth makes it clear that Yu Yevon forces the Fayth ton continue dreaming of Zanarkand -- and with the Aeons coming as a result of such memories, it is likely that everyone in Dream Zanarkand bears an appearance that has its basis in the appearance of someone from the original Zanarkand. While the appearances are based on those of the original Zanarkand's inhabitants, they are not exact and the people receiving these appearances are not the same people as those who happen to be the basis for the others' apperances. This is most likely the case with Tidus. His appearance and any superficial characteristics associated with it are drawn from Shuyin, even while Tidus as a person is wholly seperate and in no way necessarily connected to Shuyin.

On the part of the creators, Shuyin's appearance was certainly intended for the purpose of causing Yuna to believe that Tidus may yet be alive and that she could find him. This caused her to join the Gullwings, and it provided a plot twist for the game when Shuyin was revealed to be someone who most obviously wasn't Tidus. However, even from a storyline standpoint, a very clear and logical explanation -- as shown above -- can be derived to explain Tidus' resemblance to Shuyin with it leaving no fallacies and no points unaccounted for. No matter the explanation, we're, of course, left with the astounding coincedence that these two young men who would play such an important role in Spira's development -- and Yuna's life -- would have similar appearances. However, in such stories as these, amazing coincedences are to be expected, or at least excused for their contribution to the plot.

In conclusion, the most likely explanation for Tidus' resemblance to Shuyin is that Shuyin is the template upon which Tidus' physical characteristics are based, with everyone in Dream Zanarkand having someone from the original Zanarkand as their physical template, though Tidus is not at all the Dream Zanarkand version of Shuyin.
Again, note that this is Squall's work, not my own. I do agree, however, with the theory.

As to the point regarding Chappu, I would say this falls into what Squall called "amazing coincidences." In that case, however, the plot point is minor, and doesn't invite the questioning that the Shuyin/Tidus similarities did and do.