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

The mod automatically fixes bugs; even if you simply load a save file with a bug that occurred while playing without the mod, it usually fixes itself. If it doesn't fix itself, you need to hover the cursor over the buggy object (a duplicate hero, the entrance to a gray castle) and press the left Shift key + F2.
If the bug has already caused significant issues, it might not be fixable, so it's best to fix it immediately. Even better, just play with the mod and avoid these problems altogether, especially since the current settings allow you to achieve maximum comfort.