The only unit with this ability is Listrid... and you cannot select it from leveling up. The only way this overlap occurs is when one grabs a book from Pariden to give this particular unit the water ability. Not something to arise too often.

But one could place this in a different spot on the general tab like

<PerkTreePosX>2</PerkTreePosX>
<PrekTreePosY>2</PerkTreePosX>

But if it were to be located on the assassins tab then there are a lot of positions to be putting this ability.

This bug is so remote from a practical user standpoint, it barely qualifies I would call it more an annoyance.

Listrid is a level 9 champion and the only champion with this skill. It cannot be obtained by leveling up. It took me actually 25 games where I cheated to get 9 heroes before I saw this bug in action in the game. On a practical standpoint, it won't show up in a normal game.

Fixed, thanks!