Years ago, when I was editing the monthly newsletter for my parish, the rector challenged me on using the expression time and again for "repeated action." It should be time and time again, he told me. 

Now, I do not have an excess of reverence for the Collared, but this worthy had also been a graduate student in comp lit, so I took him on trust. 

The other day I was editing a text that used time and again, and I reflexively changed it to time and time again and sent it on its way. 

But then, recalling how handy the Google Ngram Viewer is, I bethought me to check it out. Time and again and time and time again were on a roughly equal footing in the 1830s but then began to diverge. Diverge sharply: time and again has been the dominant form for a century and a half.

Since it is an idiomatic expression, there is no reason whatsoever to prefer time and time again except for individual aesthetic preference, and I have been wasting my time for decades making an absolutely unnecessary change in copy. 

It is salutary for those of us who edit to periodically examine the things that Somebody Told Us which we have been applying unthinkingly. Then we can concentrate on the editing that needs to be done.