"[Paul] asked them, "Did you receive the Holy Spirit when you believed?"
They answered, "No, we have not even heard that there is a Holy Spirit."
How is it that someone can be a believer and while having never received or even heard of the Holy Spirit? Doesn't the Holy Spirit indwell all believers? The text makes clear that these were disciples, but does not make clear whose disciples they are. They were baptized once with the baptism of repentance, but the text then goes on to say that they were re-baptized in the name of the Lord Jesus.
It seems that their first baptism was inadequate and whoever discipled them left the job undone. These were nominal Christians. John's baptism was good, but it was insufficient for salvation. Some others had come before Paul to Ephesus and preached only a small part of the good news of Christ and went on their way, unknowingly leaving behind a remnant of shallow half-Christians who only think they are saved.
How much of a burden then lies on us, as members of Christ's body, to preach the whole gospel in order to make whole Christians? We have a charge to make disciples of all nations, not just to baptize but to teach them to obey all things Christ has commanded. Paul taught in the area two years, making sure all have heard the message and were rightly taught before he moved on. We ought follow him as he follows Christ.