[ForgottenMUD]

Is it truly unique, or just an extremely rare % drop?

Here are the options that I can think of:

[Delete]
Regularly clear the database from players who haven't logged for <insert time>. Perhaps something like 1 month / level. This is something that I've seen done quite often. I don't know if they are running out of space or if it's some sort of OCD perfectionism...

A bad option is to take the item from the player. Players would get very angry if the admin steals items from them.

And don't implement a rent command. Like eating and drinking, this is a highly annoying feature.

[Clone]
Re-open the quest that gives the item. Even if it's supposed to be unique, you can either make up a reason why there were several copies of the item, or just not care. Players are used to it e.g. if there's a mini-quest to save the princess, everyone runs around with her behind them, thanks to the suspension of disbelief.

[Change]
Change the game mechanics so that such situation doesn't arise again. You can randomise items as Kavir suggested, or simply make the odds of the item dropping so low that it's virtually unique. You can also open the area only once per month/year so that it cannot be farmed.

[Ignore]
Ignore the issue. Treasures get lost in real life too. The legends surrounding the item makes your MUD lore richer. The player might or might not come back but you can always create new artifacts. In fact, it's necessary if you don't want your MUD to be stagnant.