It's a good introduction to a few important aspects of how jazz evolves... first of all, the general fact that musicians steal ideas from each other. The best musicians steal the best ideas. In this way, new songs are written as variations of old songs. The degree of variation can be large or small. A common type of variation is contrafact. This means that the new song uses the same chord progression as the old song.
In the case of King Porter Stomp, one section of Morton's original composition was so appealing to jazz musicians that it became known as the "Stomp Progression," and many other songs were written using this progression. This is a common phenomenon in jazz. For example, countless other songs have been written using the chord progression to "I Got Rhythm," and any jazz musician is expected to know the "rhythm changes," the chord progression from "I Got Rhythm."
So, why is this interesting? Well, I think it's inherently interesting, but it's also interesting because it often explains why a new song seems so familiar. It can also explain how one gets two songs mixed up with each other. For dancers, it is interesting because it partly explains why we often can anticipate what is coming in an unfamiliar song. It's not just that they share a basic structure, the details of the structure-- the chord changes-- are often shared between songs.
My favorite example of this is a song based on "Blue Skies."
Listen to "Blue Skies"
Now listen to "In Walked Bud" by Thelonious Monk. It would never be mistaken for "Blue Skies," but it is a contrafact; it shares the same chords, and one can easily sing the melody to "Blue Skies" over a recording of "In Walked Bud." Try it.
"In Walked Bud"
There are endless examples of this practice in jazz... This list is only a start.
There are two other things that can cause a lot of confusion: different songs with the same name and the same songs with different names. One cannot copyright the title of a song, so anyone is free to write a new song with the same name. This can cause a lot of disappointment when you think you have found a rare version of a great song... and it turns out to be a different song entirely.
The other case is when the same song becomes known by two names. It's not clear to me why this happens; just because you give the song a new name, it does not change the rights of the person who wrote the melody. (The melody of a song may be copyrighted, but the chord changes cannot. This is why people are free to use the chord changes to make contrafacts, but one may be sued for stealing a melody.)
Here is an example of a song with two names:
So, if you hear a song, and think it sounds a lot like another song, you're probably right...