I see this theological error everywhere. Some of the giants of the faith have made it. Theology books have included this error when discussing the person of Christ. It appears in the ESV Study Bible on a chart on page 2495. I have made this mistake myself in the past. This is a very important distinction in the very words of Scripture.
Jesus is referred to (see 1 Corinthians 15:45) as the "LAST ADAM" (not the second) to show that he is not one of a possible series, but the ONLY alternative to the first Adam.
A few verses later (1 Corinthians 15:47) the word "second" does indeed occur in connection with Christ.
1st man was of the dust of the earth
2nd Man from Heaven
...and here "second Man" (not last) is very appropriate for two reasons:
1. Adam in his innocence was man as God created man, man as God intended him to be, true man. When sin entered the race, man ceased to be man as originally created (for sin always reduces rather than adds. Consequently, Jesus Christ in His sinless humanity was indeed the "second Man", the ONLY figure in history since the fall to stand as fully man in the world!
2. The whole point of His mission as the second Man "from Heaven" was that He might not be the LAST MAN, but rather, the beginning of a new series, the foundation of a new humanity, the "first born" among many brothers. (Romans 8:29)
However, His "success" as "the second Man" depends on His relation to us as "the Last Adam".
(It helps me to think of Noah as the "second Adam" because he was the one God gave the same charge to replenish the earth - note: the Bible never calls Noah the second Adam, it is just to help me remember that Noah fulfilled this role already.)
A full treatment of this is in chapter 14 of "The Glory of Christ" by Peter Lewis. (published by Moody)