Man am I glad I ain't the only one plagued with this. I too try to spred similar tunes out to avoid this. Usualy it happens when I am trying something new but this time it was stuff in my standard material. I once had it happen with "Bury Me Beneath the Willows" & "This Land is Your Land".
For me it is most aggravating when I mix tunes. If it happens with lyrics I remember how the Weavers used to use the same lyrics over & over for laughs like mixing "Irene Goodnight" with "Go Tell Aunt Rhodie."
Often I'll forget a verse or mix two together or switch them around. Most people don't catch it but they do with tunes.