Is there an association between polydactyly (having extra digits) and "double rows of teeth" in accounts of giants? Setting aside for a moment the issue of whether giants are actually real, let's just focus on whether this often-repeated association between extra fingers and extra teeth has any basis.
Until someone proves me wrong, my answer is "no:" there is no association between polydactyly and "double rows of teeth" in the accounts of giants.
I can't recall a single instance that I've seen of a report of a giant skeleton from North America (or anywhere else) that specifies that both a "double row of teeth" and "six fingers" were present. I think these two "traits" have gotten welded together by the uncritical imagination of giant enthusiasts. The "and" joining these two sets of traits should actually be an "or."
I've argued that the phrases "double rows of teeth" and "double teeth all around" weren't intended, in most cases, to actually indicate that multiple, layered rows of teeth were present. I think the use of those phrases is related to 19th century idioms used in American English.
Conversely, the "six fingers" reference apparently comes primarily from the biblical description of Goliath (1 Chronicles 20:6):
"And yet again there was war at Gath, where was a man of great stature, whose fingers and toes were four and twenty, six on each hand, and six on each foot: and he also was the son of the giant."
Did Goliath, the most famous six-fingered giant, have "double rows of teeth"? Well, let's ask T. M. Sparks in his book Giants: The Amazing Truth (chapter 1):
"Now these children were not part of God's creation, they were mutated forms of life. They were giants of our past; they would have six fingers and six toes on each limb. They also had double rows of teeth. Goliath came from this lineage having 6 fingers and 6 toes also. His teeth were not mentioned in Scripture, only the toes and fingers of his brothers were. But I am sure he had this distinction as well."
So there you have it: the Bible doesn't actually say it using actual words, but it's surely true . . . because everyone knows giants have six fingers, six toes, and double rows of teeth, right?
No, not really. That's a swing and a miss if you care about evidence.
I remember I heard once that there was a giant who had a goose that laid golden eggs. So maybe we should just throw that in there with Goliath also.
Let's make this a falsifiable hypothesis. I'll state there is no primary account of a giant (i.e., neither a description of a skeleton nor a historical text) that actually specifies the presence of both polydactyly and "double rows of teeth." Can any of you falsify that hypothesis and point me to a primary source that describes a giant with both polydactyly and "double rows of teeth"? If you can, then we can talk about how common the association is. But let's first just start with one.
Until then, I think this association between "double rows of teeth" and "six fingers" is another thing to throw into the category of a modern myth.