Using arrow diagrams If 7 dice are rolled, and explain why must two at least be the same value?

Because dice is six sided, so first six dice if all get numbers 1-6 in first 6 role , there is no number left for outcome of 7th dice to be different than the 1-6

If first six dices show the same number on at least 2 dice, then criteria for 2 die come up with minimum twice the same number is met.