Using magic twice in one round is considered a bug. I suggest not using it. If you bypass this bug, I will ask you to find a way to justify it :) "like, let's let Axe use magic twice too" - that is at your discretion.

The justification is this - if it is allowed for EVERYONE, then there is no need to check anyone regarding the use of magic twice per round. That's how those three characters I played hot-seat with explained it to me. In this case, all participants are at their computers under the same conditions and are all equal to one another. Like, for double usage, everyone spends twice as many mana points per round.

If you believe you have the right, you may use it... A way to check your Walkthrough in detail exists.

I don't "believe" anything; I am describing the situation in detail. Yes, I know there is a way to check the leader's Walkthrough in detail, but not every participant's. For this, a most detailed description is needed, including the mana points of EACH hero before EACH battle, plus how they cast in each battle, and how much was left at the end of the battle. Additionally, saves are needed not only for the start and end of the day, but also for the start and end of every battle. This is not in the rules!
1. Agree - making such a description for a map like this might take so long you'd die before finishing :)

In case of doubt, any player has the full right to demand a detailed description of your Walkthrough from you, and there is a possibility to calculate your run step-by-step. And you know this...

That is exactly what I am unsure about. Because what you wrote is not in the rules. Specifically, no one except the judge has the right to demand anything from you; they can ask, but you have the right to refuse. And it's unlikely the judge will manage to calculate every person's run step-by-step, at best only the leader's. But imagine if the judge tells someone to whip up a description specifying the mana points of each hero before every battle, a description of every battle, how much was left, and to send all those saves - THAT IS UNREAL, especially on a map like this. Well, what if this is used not by the leader, but by a player who took 3rd place - NO ONE WILL CHECK THEM AT ALL! (By this, I mean a DETAILED check). In this case, an injustice toward other players remains.

I still hope it doesn't come to that.

Come to what? To a step-by-step check of saves based on detailed descriptions? Yes, I am simply sure of that!
The simplest way is to allow EVERYONE what was once forbidden by someone, but the reasons for the prohibition were not explained, nor was a way to effectively verify the use of this bug found :)

You apparently haven't read the thread about the OFF Heroes 1 Tournaments. There, I proposed a point on strengthening the checks at least for the leaders' Walkthroughs, but the proposal was not accepted. So read the rules carefully. If you personally, for example, take 3rd place, you cannot personally check the player in 2nd place, especially if they use double magic not in battles with legions but in battles with dozens. Such battles won't even be mentioned in descriptions on this map! I am surprised that you specifically had to have this explained to you :(

Added after 12 minutes
Amber Soler, do you want to be convinced that you are wrong? :) Officially allow everyone to use magic twice ONLY in ONE battle with any of the heroes. After the tournament, let the judge or YOU say who used magic twice in which battle, or at least identify just one "unclean" battle of the leader. Your correct answers would convince me that I am wrong.
PS. One can, of course, think that everyone plays fair on their word, but I don't want to play and think that I might end up looking like a bit of a fool.

Added after 56 minutes
I'll repeat the essence of my proposal so you don't have to dig and look for it in another thread. Officially allow the player who took 2nd place to check the player who took 1st, allow the player who took 3rd to check the 2nd place player, and the 4th to check the 3rd, respectively. The Judge can check anyone, regardless of what place they took themselves.
The chain ends there. In this way, the top three leaders can be checked in detail, which also helps the judge during verification. If the leaders' results are verified this way and everything is clean, is there any point in checking the other players in detail? I think not. If this point had been introduced into the rules, then it would be one thing, and double casting could be forbidden. But a prohibition without corresponding verification makes no sense.