In its closing Philemon mentions these figures:
23 Epaphras, my fellow prisoner in Christ Jesus, greets you, 24 as do Mark, Aristarchus, Demas, and Luke, my fellow workers.
These same people show up again in Colossians:
10 Aristarchus, my fellow prisoner, sends you his greetings; and also Barnabas’ cousin Mark (about whom you received instructions; if he comes to you, welcome him); 11 and also Jesus who is called Justus; these are the only fellow workers for the kingdom of God who are from the circumcision, and they have proved to be an encouragement to me. 12 Epaphras, who is one of your own, a bond-servant of Christ Jesus, sends you his greetings, always striving earnestly for you in his prayers, that you may stand mature and fully assured in all the will of God. 13 For I testify for him that he has a deep concern for you and for those who are in Laodicea and Hierapolis. 14 Luke, the beloved physician, sends you his greetings, and Demas does also. 15 Greet the brothers and sisters who are in Laodicea and also Nympha and the church that is in her house. 16 When this letter is read among you, have it also read in the church of the Laodiceans; and you, for your part, read [m]my letter that is coming from Laodicea. 17 Tell Archippus, “See to the ministry which you have received in the Lord, so that you may fulfill it.”
Philemon: 17 If then you regard me as a partner, accept him as you would me. 18 But if he has wronged you in any way or owes you anything, charge that to my account; 19 I, Paul, have written this with my own hand, I will repay it (not to mention to you that you owe to me even your own self as well). 20 Yes, brother, let me benefit from you in the Lord; refresh my heart in Christ.
Colossians 4: 18 I, Paul, write this greeting with my own hand. Remember my imprisonment. Grace be with you.
So, what's up with this? Both are supposedly written from prison. Both mention the same set of people. Why is Philemon considered authentic?