Juggernaut must attack each turn if able. Dueling Grounds only allows one creature to attack each turn. What happens if a player has multiple Juggernauts and Dueling Grounds in play?
I'm thinking there are a couple options here. The first is that rule 717.1 applies, which reads:
717.1. If a player takes an illegal action or starts to take an action but can't legally complete it, the entire action is reversed and any payments already made are canceled. [...]
This rule would indicate that both Juggernauts become untapped and neither are declared as attackers. If this is the case, is the whole Declare Attackers step reversed? The illegal action occurs as part of rule 508.1 at the beginning of the game:
508.1. First, the active player declares attackers. This turn-based action doesn't use the stack. To declare attackers, the active player follows the steps below, in order. If at any point during the declaration of attackers, the active player is unable to comply with any of the steps listed below, the declaration is illegal; the game returns to the moment before the declaration (see rule 717, "Handling Illegal Actions"). (emph. mine)
Because the player declares two attackers as part of 508.1, but the player isn't allowed to, this seems to indicate that the game returns to pre-508.1 conditions. But if this is the case, does the player get to declare another attacker?
My intuition tells me that the Active Player must declare both Juggernauts as an attack, which is illegal and is reversed, sending the phase back to the beginning of Declare Attackers. At this point, they could declare one of the two Juggernauts as an attacker. However, this is an inordinately complicated interaction, and I have very little confidence that I'm right.
Could somebody possibly clarify what happens in this situation?