I'm not sure where I picked it up.
And unless I consciously stop and catch it, I'll also use an extra comma in lists. For example, I'll say: "We looked into our first, second, and third options in this situation."
100 years ago, I would have been bang-on correct, too!
To my knowledge, both are gramatically correct. One might be more archaic or obsolete (extra comma), but I think both are correct nonetheless.
Then again, to-morrow is obsolete and probably gramatically incorrect nowadays, so I dunno. I wonder who decides when a word, spelling or matter of grammer is no longer technically correct?
