There is no contradiction here. The Bible is the inerrant Word of God and all such ‘contradictions’ are pointed out in ignorance or because the matter is a complicated one requiring knowledge of history and other matters or simply because some things do not translate well into English. Apologists have dealt with them all for years.
Adam and Eve are the first human beings created on the 6th Day by God as they are and all of humanity is descended form them. The Church also recognizes Adam and Eve as real individuals as the Bible teaches. Christ also referred to them as real people and his lineage can be traced back to them being as how Christ is the New Adam.
In a perfect world before the Fall between only Adam and Eve, incest would be the only way for the world to be populated. This can find grounds in science as today incest is taboo because of genetic problems. In paradise and for the recent ages after the Fall the genetic problems of Adam and Cain etc were either non existant or not that big a problem. However as the universe continues to age and die, genetic material also degrades and so God gave a clear commandment against incest in the time of Moses. Given that the Pentateuch would be authored by Moses which includes the Book of Genesis, Moses would downplay references to incest.
For more information I recommend this article:
Where did Cain get his wife?
answersingenesis.org/home/area/tools/cains_wife.asp