BTW, sometimes melodies have the same chord progression and/or are very similar but not identical. E.g., This Land Is Your Land and You Are My Sunshine or Goodnight, Irene and Roll On Columbia.   This is the case with Lennon's "So This Is Christmas" and "Stewball."   Aside from the chorus/bridge of STIC, even the verses are not identical in melody.    Transcribe the "dots" and you'll see that pretty easily.