More examples of possibly unknown copying:

The first five notes of Flower of Scotland are identical to the start of Verdi's Chorus of the Hebrew Slaves. (We have played both back-to-back in a session!)

Someone started singing the Burns song John Anderson, my Jo: he continued, and knowing smiles and stifled sniggers went round as he unwittingly morphed the tune into Ghost Riders in the Sky!

A songwriting friend launched a "new" song on us the other week. It had pretty much the same tune as Devonshire Carol by John Tams: he said he had never heard the latter. (I told him privately later that I thought there was a resemblance, and he went away and "discovered" John Tams, admitted the uncanny likeness in tunes, and changed his own).