He had a fever, and he was coming from a country that has an Ebola outbreak. He also was in contact with a person who had recently died from Ebola. He lied about this. Had the government been cautious and serious about protecting America, he would have never been allowed into the US.
Duncan was presumably exposed on the 15th, entered the US on the 20, and began showing symptoms on the 24th.
Given the usual progression of the disease, it’s unlikely he had a fever on the 20th.
Now he probably knew he’d been exposed, though there is some question about that, and had he been honest about his recent medical history he would have been stopped before entering the country.
But I’ve never seen any evidence at all that US procedures were not properly followed. Those procedures just wouldn’t have been effective at keeping him out.
Now quite arguably those procedures should have been very different, but the government didn’t blow Duncan’s case off. They followed procedure.
Just as, BTW, the screeners did on 9/11. Both cases were a case of inadequate policy, not failure to follow policy.