What if the user typed the new value, then re-typed? The wrong new value would be still in the list...

Well, I suppose you could nag the suer and display an "Are you sure?" messagebox? However, in my experience, users just get used to being annoyed like this and hit < ENTER > without even reading the message.

This is just one of those scenarios that is "Valid but wrong". And for this, there is no software solution < s >.