Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)

Roll-off
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If the rules require players to roll-off, this simply means that each player rolls a dice and the player that scores the highest result wins the roll-off. If the players roll the same result, both dice must be re-rolled again until one player is the winner – any modifiers that applied to the first dice roll are also applied to any further rolls.

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