Maybe it's such a short little word that we subconsciously imagine two that are better than one. Or perhaps it's the way it reminds us of one of the first words that a baby learns, and we were all babies once. Or, more likely, it's because after we type it, we often pause to check the name that comes next and then forget we've already done it and type it again. I'm really not sure why it is, but By by (for by) is an exceedingly common typo—there were 582 of them in OhioLINK alone. By the by, this one qualifies as a "high probability" typo on the Ballard list and has never been blogged about before, so by golly, it's high time it's given its due. Of course, considering that the word by occurs in many, if not most, bibliographic records, there are bound to be quite a few of these errors in your catalogs*. Like leaving a good party (or even a bad one), it may take you a little while, but try and say "Bye bye" to every one you find there.
*Note that "by" is often treated as a "stopword" and that different systems require different methods of phrase searching, so you may need to first figure out how to search for this kind of typo in your own database. In mine, I had to write [single quote, double quote] by by [double quote, single quote].
(Cast members in the 2009 production of Bye Bye Birdie at the Regals Musical Society in Sydney, Australia, from Wikimedia Commons.)