The exposition of this passage is much more direct, like a one-two knock out. Paul begins his confrontation with great speed and forcefulness. My exegesis will be short for the sake of time and the flow of the passage
Every translation of the first sentence of this paragraph relates Paul's sarcastic surprise at the Galatians. Like all sarcasm, Paul is truthful and facetious. From my analysis, it appears he is truthfully concerned of the weight of their choices. He is also ridiculing that which they are choosing, namely the "other gospel". The gravity of the their choice is one of defecting to an enemy force. The word often translated "deserting" is also translated "to remove" or "to transfer allegiance." They are replacing one with the other, as if they were equal.
But Paul makes clear they are not equal. He calls this doctrine they are accepting in place of their first faith a "different" one, as one in a sequence. If you were counting different messages like T.V. channels, it is just one more out of the endless cable programing. It is a "different" gospel but it is not "another" one. When he says "not another" one, he is saying "not one of the same quality". I might tell you about many stones but very few will be of the same quality of the rare "Blue Heart" diamond.
The end purpose of those providing this alternative message to the Galatians is clear. They see an opportunity to take advantage of them like a street peddler. What they are peddling though is far worse than a man offering fake Rolex watches. They are peddling with life and death--eternity. They are like a voodoo doctor dressing up in White Coat and coming into the cancer wing of the hospital. They go up to everyone there and write discharge papers telling them they can be free of cancer if they sign up for their health plan.
Paul storms into this situation and gives a double curse upon these men. This curse is very controversial and I will not pretend to have considered every possible position. There are many words for curse in the bible and many consequences of such. This double curse seems to be the heaviest of those.
Many may think of curse, as that which is using a serious words in "unacceptable" statements or situations (e.g. "What the Hell?", "Damn it!", "Oh my God!", "Jesus Christ!"). The Bible addresses these in various ways that we will not go into. In general these would be curses of "making light" of important things, such as God's name. That's not what Paul is refering too. This is not a curse these men are uttering; they are being placed under a curse.
However, Paul is in a sense saying "Damn" those who give pretend to have another message from God. The curse is more liken to the one in Deut 27:18: "Cursed is the man who leads a blind man on[to] the road." It is a curse of consequence. It is the curse God gave to Adam and Eve as He cast them out of the garden to make their work difficult and their childbearing painful and their physical lives mortal. The accursed is destined for God's hand to be against him and, if never removed, to crush him.
What is scarier, this knock-out that Paul gives with repeating this curse twice is coming from God. God is the one who will "punch" these twisted men with a curse.