Can a thief steal if he can't equip the item?
From the rules:

All Items you have in play are considered &quot;carried.&quot; Items that are actually giving you a bonus are &quot;equipped.&quot;
Anyone can carry any Item (except for extra Big items), but you may equip only one Headgear, [etc.] ... If you are carrying two Headgear cards, for instance,
you can equip only one of them at a time.

This means that even if an item can't be equipped (because you already have an item equipped in that slot, or any other reason), it can still be carried, so a thief can still steal it and just carry it.