When the monsters attack in 2003, how do you make the monster sprite dissapear so there won't be two monsters?

I'm not sure I'm fully understanding your question. You'll need to define a little more. Is it as a skill? or maybe a battle system? or just general attacking?

do you mean when the monster attacks using a animation (assuming that you used a battle animation for the attack of the same sprite
as the monster), that the monster is still there?
(does this make sense?)

I'm talking about animations attacking.