This is where a phrase is repeated to show where an author starts a unit, and where he finishes a unit.
Matthew 19:30 But many who are first will be last, and the last first.
It would be possible to see this as the concluding words to what Jesus has said previously. In which case when studying the text, we might be tempted to stop right there, and even in a sermon we might be tempted to end the sermon at that verse.
However if we read on, we see the same phrase used again in the next chapter,
Matthew 20:16 So the last will be first, and the first last.
This is called inclusion, or if you want to sound clever, inclusio.
I found this very helpful when I was preparing a sermon on this last week. It helped me to decide how much of the text I was going to teach. I started the sermon at Mat 13:27, and ended it at Mat 20:16. This meant that I was able to teach a unit of thought that Matthew put there, rather than a chapter division that someone put there hundreds of years later.