You just learned a valuable lesson homie lol. Gotta always get the weapon you want first, then build your stats to use said weapon. Otherwise you'll get near the end and say "sh1t now I gotta go all the way through NG+ just to get it again" lol. Besides, not a lot of people are able and/or willing to give up a boss soul weapon. Usually when someone already has the equipment, they just take the souls.