HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

1.

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And further, the dialogue is about the HD version's abnormal duplication, which has no analogue in SoD? Please clarify.

In this case, saves are not needed: the situation is modeled. One hero stands on one side of the entrance/exit to the Dungeon (this is the "metro" :)), and the second hero transfers troops to him from the other side of the gate, and the duplication occurs during contact. Something similar, I think, also happens in the Monolith.

2. Regarding transferring troops from an ally during his turn, it is more difficult to repeat the situation, as an online game is required.

3.

it will be necessary to make it possible to autosave several turns in HD...

And then, can you make autosaves for all turns? At the beginning and end of the turn? With a record in the title of the players' names and the game day number? In general, integrate an autosaver that players use as a separate application (which, by the way, can also save saves in an unpacked form so that after the end of the game, the entire package of saves can be archived with a minimum byte size). Since autosaves... whether it's a few turns or just one day, it's essentially the same thing. In online play, saves for all days are important - for analysis and verification of the game, for creating replays, for rolling back if a bug or disconnection occurs.

4.

I made a market call from the adventure screen and from the city window (where the market is not built), I wanted to assign this to the "M" or "T" keys, but they turned out to be occupied. I decided to assign it to "B", but maybe you can suggest a more convenient or logical option?

There is another delicate moment here. Calling the market with a hotkey... it sounds somewhat alarming. For offline play, this option is quite harmless, but in online play, the situation will not be in favor of those who do not have this button. Although the time saving will be effective, because we use the market often and a lot.