Why does Matthew "double" people in his gospel account? Three times in his gospel, he includes narratives where two people are healed, and he also mentions a donkey and a colt in the triumphal entry account. Two demoniacs are healed in 8:28-34, two blind men are healed in 9:27-31, another set of blind men are healed in 20:29-21:27, and the donkey and colt are mentioned in 21:2. If the four-source theory is correct (By the way, could you mention if you support the four-source theory, the Griesback theory, or another theory about sources?), Matthew would be aware that Mark parallels three of these passages, only mentioning one demoniac, one blind man (Bartimaeus), and one colt. I'm thinking that there has to be some literary or theological purpose behind it because it occurs four times. Matthew seems to be interested in numbers in some way. This can be seen in his formation of the genealogy. What do you think about this, and what have some other scholars written about this?
This is a good set of questions, and, in my experience, everyone who gets serious about the Bible (and starts paying attention to it) inevitable asks about such things (although I must say that you have certainly delved deeper than most).
To begin, I have always found, that, in the end, there are no contradictions in the scriptures. There are many things which are difficult to understand at first, but, again in my experience, this is always a case of us trying to gradually bring our understanding up to speed. To understand the whole Bible in all its depth and glory is a life-long task. It shouldn't be surprising if there are a number of areas that won't resolve for us until after much study - and often it is something seemingly unrelated in scripture elsewhere that opens up a passage that has had us scratching our heads. The one thing of critical importance is to keep faith with scripture, always remembering that it is the truth (and without blemish in its truthfulness), if only we can come to understand it aright (the text, the context, the language, the grammar, the theology, the history, etc.). First principle: the Bible doesn't have to explain itself to us; we have to work to understand it on its own terms. If we do, we will have many experiences where different passages may have seemed problematic in the beginning, but come to be thoroughly understandable and understood in the end (see: Read Your Bible: a basic Christian right and responsibility).
As to the specifics of your questions, the gospels do indeed form a special case (and perhaps for that reason have come in for more subtle attack than much of the rest of scripture). Given the critical importance of knowing about our Lord, the One who came and died for us, the One upon whom our entire faith, our entire lives is based, it was of critical importance that Jesus' life and teachings be documented by a multiplicity of witnesses (cf. 2Cor.13:1). As also is true of legal proceedings, however, that does not mean that all witnesses will tell their story in the same way or from the same point of view or include the same details - but (again, as in court) the fact that different witnesses may approach their stories in different ways and include different details does not necessarily mean that they are being inaccurate. When it comes to the inspired Word of God, this is doubly true.
This is a long-winded way of saying that the "doubling" in Matthew is only apparently a problem. The long-standing and widely accepted theory often referred to as "the priority of Mark" is wrong (along with many such theories and variations which completely ignore the biblical teaching of inspiration in favor of mundane human composition theories: cf. 2Pet.1:19-21). Matthew and Mark both, along with Luke and John, all wrote their witnesses to the life and ministry of our Lord by the will of God, inspired by the Spirit of God (2Tim.3:16; cf. Jn.14:26). This was recognized even at the time, as the case of Paul's quotation of Luke as scripture makes clear (1Tim.5:18 with Lk.10:7; cf. Peter's equivalent treatment of Paul's epistles: 2Pet.3:15-16). Therefore the differences in their accounts must be sought in the different secondary purposes of their gospels (within the overarching main one of telling the story of our Lord).
To make an ever lengthening story somewhat shorter, I believe that Mark, a protégé of Peter, wrote under Peter's apostolic authority (as Luke did for/with Paul) to meet the particular need of having a "basic" gospel for the initial instruction of gentile converts. Matthew wrote first, but wrote for a largely Jewish audience. And while we in the modern west who have been raised in a Judeo-Christian milieu may have only minor trouble digesting the detailed information on Jewish culture that permeates Matthew, for uninitiated former pagans of Rome who knew almost nothing about Jews or Jewish customs or the Old Testament, many of the things in Matthew would have been "not the best place to start". Analogously, while I thrive on the OT, I tell new converts to start with the NT and the Psalms - the last thing a new believer needs is to "start at the beginning" and get bogged down in Leviticus when he/she has never read the epistles. Similarly, there was a need to provide a shorter version of the gospel which avoided many of the things that might "bog down" a new gentile convert, raising questions about Jewish particulars for which he/she at the beginning had no frame of reference whatsoever. Genealogy, which you mention, is a good example. This is a very important issue, of course, but one which it is very difficult to explain without explaining Genesis and the Davidic line (and we're back into the OT).
This brings us right to your "doubling" question. In my view, most of these instances are not contradictory. There were two, so Matthew gives two. But the fact of "two" raises questions which a concentration on the more significant "one" in Mark alleviates (getting to the point without complicating things for the new convert: a valid technique with God's apparent stamp of approval). Take the donkey and the colt. Yes there were the two, but of course Jesus rode one. And why two? To fulfill in every respect in a way that would be unmistakable the Zechariah prophecy, an unmistakable sign to all who watched Him ride into Jerusalem that day on a donkey's colt with a second donkey in tow (Zech.9:9; and cf. Gen.49:11). The two mounts are also symbolic of the two advents of our Lord (cf., Rev.19:11; and possibly also of the soon to included gentile portion of the Church added to Israel: Peter was ministering to a gentile audience). Jesus rode into Jerusalem once as the suffering Servant, and at the Second Advent He will come on a white horse as the conquering Messiah (exactly what the crowd who welcomed Him with hosannas was expecting in 33 A.D.). But to include all this in Mark meant explaining the issues involved in interpreting Old Testament prophecy and explaining the expectations of Jewish believers at the time of His crucifixion, both of which were likewise completely foreign to the Roman mind (they still sometimes cause difficulties for new Christians today).
So for shortness, simplicity, and to avoid making issues of things which, while important, were not as important as the basic message of the gospel, Mark was written as it was – the truth put in a way to make truth the issue.
This is, of course, only one aspect of Mark (this picture too is simplified!), wherein we find a truly important and blessed book in its own right, giving us things from Peter's perspective, and providing another wonderful witness to the life and the words of our dear Lord.
Finally, on the two demoniacs in Matthew, Luke has only one, and this seems to be the one who responded to our Lord's merciful deliverance by putting his faith in Him. Not everyone delivered by our Lord did so, after all (cf. only one of the ten cleansed lepers responding: Lk.17:7), so focusing on the believer(s) instead of the whole contingent is another reason for the different treatments.
Here are some other links that bear on this question:
The Life of Christ (in BB 4A: Christology)
Gospel Questions I
Gospel Question II
Gospel Questions III
Some Greek Questions in the Gospels (John 1:3; 2:19; 8:58; Luke 23:43)
Enoch's Walk with God and Some Questions in the Gospels.
How can we know whose interpretation of the Bible is right (II)?
How can we know whose interpretation of the Bible is right (I)?
Hope this is of some help to you.
Yours in Him who is the Truth, our Lord Jesus Christ.