I happened to be flipping through some old Action issues today and noticed this trend. In Action #254-255 we have the first appearance of the adult Bizarro Superman. In Action #256, we see the Superman of the future. In Action #257, Clark Kent becomes a "second" Superman:
In #258, Superman encounters Cosmic Man:
Who turns out to be a robot. In Action #259, the other Superman is Superboy:
After a few issues off (#260-262), the Bizarros return for another two-parter in #263-264. It's Hyper-Man's turn in Action #265:
Then in #267-268, there's a two parter featuring a Superman of the past:
After that, the stories mostly get back to normal, but it's striking that there were so many tales with a similar theme over the course of about a year and a half.