This question already has an answer here:
Continuing the response to the meta call for contradiction.
In Genesis 22, we find the following passage:
And it was after these events that the God tested Abraham; and said to him "Abraham", who replied "Here I am." And said "Take now your son, your only, which you have loved, Isaac, and go you to the land of Moria, and raise him up there as an offering, on one of the mountains, which I will tell you."
Why is Isaac called Abraham's only son, when Ishmael would be the firstborn? It is traditional to enumerate the sons of the slave wives as belonging to the proper wife (see later Genesis regarding the sons of Jacob through slaves belonging to Leah/Rachel--- Dan is still the first generated of Rachel, even though he is born by Bilha), so Hagar's son would count. But in this verse, it is as if Abraham only has one son.