playBunny wrote:  "If one of the dice is always the same and the other is a fair roll then
duplication of both would occur with a frequency of 1/6 rather than the
1/18 that's expected, so 3 times more often."

Some error like that could explain all the other figures seen so far: the excessive exact matches, the excessive near misses, etc.

And IF in 1/2 (exactly or approximately) of cases, 1 of the opener's dice is being "re-used" to generate responder's roll...we would overall also expect to see (exactly or approximately) 1/2 the number of expected cases where BOTH of responders dice differ from opener's roll.

And that is in fact what the data showed for my 2009 games:
Average expectation of responder's dice both differing from opener's dice out of 137 played = 60.888_ games.
Observed number=28 games