The question of free will is a thorny one, full of hazards and pitfalls, and many wayfarers have been snared. It's ultimately a philosophical question. To what extent do we freely choose our destiny? To what extent is our destiny, or at least the parameters of choice, determined? This philosophical problem becomes a theological problem when one throws God in the mix. In the end Christianity and the other Abrahamic faiths before and after it conclude with the paradox: it's both. Some have emphasized one over the other perhaps. I suppose the extreme form of "double" predestination (often associated with Augustine and Calvin) is that those destined to heaven or hell is predetermined in the mind of God before the foundation of the world. Calvin called this a "decretum horribile," the horrible decree. It's clearly an unpleasant thought and seems unfair, absurd, even malevolent. The basic reason why someone would take this view—and of course they always put themselves in the "saints" (Elect) not the "aints" (Reprobate) category—is to preserve the basic concept of God. God is omniscient and omnipotent. That being the case, how could anything ever happen in this world that God didn't know about? If God is ever "surprised" by something that we do, some choice that we make from our free will, then God would not be God, as we understand the term. It's an attempt to be logical and apologetic to the Christian faith, but such logic comes at a price.