How many different lines with all the 28 dominoes, following the usual rules. That is two dominoes touch by the same number of points ?

The dominoes are in a line from left to right, the same string "reversed" considered as different.

(The usage says the doubles are "across", this doesn't change anything to the problem)

You may practice first with a reduced set from 0-0 to 4-4

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Or even reduced further, from 0-0 to 2-2 !

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before dealing with the full set from 0-0 to 6-6

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What do you think about an extended set from 0-0 to 9-9 ?

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