Mumms the word?

When did doubling the end consonant before adding an s become acceptable?

There is a rule for doubling the vowel at the end of a word when adding an ending.  It seems like a complicated rule (double the final consonant if it is a single consonant preceded by a single vowel and the ending you're adding begins in a vowel), and if you don't know it or remember it intuitively from years of spelling in elementary school, using a spell-checking program is a good idea for you.

It seems to me that Meijer sign makers would have a spell checker at their disposal.  Especially since this sign was obviously typed on some sort of computer.