Using the default bindings set when choosing the Steam Deck profile on setup has a duplicate binding when looking at the BlizzardUI > Options > Keybindings.

LT+RT+☰ (hamburger) is bound to "Interface Panel > Toggle Spellbook"
RT+LT+☰ is bound to "Camera > Zoom In"

Using the keys provides the Zoom In function but not the Spellbook function. I'm assuming LT+RT is identical to RT+LT as far as ability for WoW to see the modifiers.

(may be a generic issue to other controllers, untested)

...

In ConsolePort's bindings menu the same duplication appears.

The logical change would be for LT+☰ to be the Zoom In ... however:

LT+☰ is bound to "Toggle Raid Cursor"

Note: I can't find "Toggle Raid Cursor" in the Blizzard keybinds, only in ConsolePort ... in fact I've never seen that element appear before until I used LT+☰.

I'm not sure how often it gets used but it is really interesting that there's an option not available through the default bindings UI. What is the main purpose? Regardless the Zoom Out function might be the better choice for it as a default.

The default set does not have a binding for Toggle Spell Book. A binding cannot be duplicate. What you're seeing is likely an Alt combo while not having Alt mapped as a modifier. There is nothing preventing this from happening, since you may combine with Alt without mapping it as a modifier. From ConsolePort's point of view, bindings with unmapped modifiers do not have proper icons (since it doesn't know what the modifier is supposed to be mapped to) and may appear as duplicate of some other binding.