Critical Strike chance is rolled twice when determining if a Critical Strike should occur:
If either roll succeeds the hit will be a regular Critical Strike, and if both rolls succeed the amount of extra damage dealt by the Critical Strike will be doubled. This is equivalent to doubling the amount by which the Critical Strike Multiplier exceeds 100 for that Hit.

If a modifier makes your Critical Strike Chance Lucky or Unlucky, that luck will apply individually to each of the critical strike chance rolls.

Effects which cause Hits to Always be Critical Strikes prevent Bifurcation - those hits do not roll the chance, so cannot roll it twice.