Guild Wars 2

I add a sigil to a weapon and I can no longer use the pvp slot in the locker for it.

yes you can. right click on the item -> deposit collectible. Deposit it manually, deposit all collectibles etc. im not sure how you came to this conclusion?

if you can’t deposit is because you have an equal item deposited already (armour/weapon)

Yeah its likely you already have an item using that slot – i get it often when i get more and more of the same useless crap i just salvage it.