Grammar Geek wrote:

It's highly nonstandard in my experience. Granted, different countries have different styles. You say that "The Davis's are all coming to dinner" is the correct way to write something that says that the entire Davis family is coming?

It is a standard variant, according to the Oxford Companion to the English Language. You'll have to take it up with them.

Cool Breeze wrote:

Alienvoord wrote: You don't have to agree, but the fact is that these are standard uses. If a noun ends in s, the standard plural ending is es. It makes no difference whether the noun is a common noun or a proper noun: She bought a dress. She bought two dresses. (NOT: dress's) CB

Yes, but with family names, apostrophe-s is a standard variant: Jones's. Altho this is not as common as the variant without the apostrophe.

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There was formerly a respectable tradition (17-19c) of using the apostrophe for noun plurals, especially in loanwords ending in a vowel (as in We do confess Errata's, Leonard Lichfield, 1641, and Comma's are used, Philip Luckcombe, 1771) and in the consonants s, z, ch, sh, (as in waltz's and cotillions, Washington Irving, 1804). Although this practice is rare in 20c standard usage, the apostrophe of plurality continues in at least five areas: (1) with abbreviations such as V.I.P.'s or VIP's, although such forms as VIPs are now widespread. (2) With letters of the alphabet, as in His i's are just like his a's and Dot your i's and cross your t's. In the phrase do's and don'ts, the apostrophe of plurality occurs in the first word but not the second, which has the apostrophe of omission: by and large, the use of two apostrophes close together (as in don't's) is avoided. (3) In decade dates, such as the 1980's, although such apostrophe-free forms as the 1980s are widespread, as are such truncations as the '80s, the form the '80's being unlikely. (4) In family names, especially if they end in -s, as in keeping up with the Jones's, as opposed to the Joneses, a form that is also common. (5) in the non-standard ('illiterate') use often called in BrE the greengrocer's apostrophe, as in apple's 55p per lb and We sell the original shepherds pie's (notice in a shop window, Canterbury, England).
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- The Oxford Companion to the English Language page 75

What's so wrong with some variation anyway?

You're welcome!

I wouldn't say they have an aversion to two apostrophes in one word. I think they're just describing the fact that we usually don't put two apostrophes in one word - except for fo'c'sle.