One-of-a-kind items are not handled

[Silwhoon]

Players are only allowed to have 1 one-of-a-kind item in their inventory (including if it's equipped). At the moment this is not the case

[doriyan13]

for equips it's utilized by the "only" property, need to take it to account when trying to add equip (it's the same for items but i need to handle this property there)