Paul's return to Ephesus presents him with a unique situation in the New Testament: the re-baptizing of believers who were not aware that the Holy Spirit was available to them.  This event serves as a good reminder that the book of Acts represents a shifting of the administration from the Old Covenant to the New.  Also, after having an extended time in the Synagogue with the Jews, Paul shifts his preaching venue to the School of Tyrannus, legitimizing it for the Gentiles and establishing Ephesus as the cradle of Christianity in Asia Minor.

[Act 19:1-10 NASB] 1 It happened that while Apollos was at Corinth, Paul passed through the upper country and came to Ephesus, and found some disciples. 2 He said to them, "Did you receive the Holy Spirit when you believed?" And they [said] to him, "No, we have not even heard whether there is a Holy Spirit." 3 And he said, "Into what then were you baptized?" And they said, "Into John's baptism." 4 Paul said, "John baptized with the baptism of repentance, telling the people to believe in Him who was coming after him, that is, in Jesus." 5 When they heard this, they were baptized in the name of the Lord Jesus. 6 And when Paul had laid his hands upon them, the Holy Spirit came on them, and they [began] speaking with tongues and prophesying. 7 There were in all about twelve men. 8 And he entered the synagogue and continued speaking out boldly for three months, reasoning and persuading [them] about the kingdom of God. 9 But when some were becoming hardened and disobedient, speaking evil of the Way before the people, he withdrew from them and took away the disciples, reasoning daily in the school of Tyrannus. 10 This took place for two years, so that all who lived in Asia heard the word of the Lord, both Jews and Greeks.