Q: Email address is already associated with another Apple ID even though I deleted that email address from the Apple ID

My step daughter accidentally had two different Apple IDs set up for her two devices. Her Gmail account was associated with the Apple ID that she doesn't want to use, so I replaced that email address with another one we're not using. I then tried to associate her Gmail address with the Apple ID that she is using and I keep receiving the error message that the Gmail address is already associated with an Apple ID. Why am I receiving this message if it isn't correct?