This warning will populate when you are trying to add an ISBN to your account that is already existed - if you can't see that on your account, it may be one that you have deleted from your account.
When deleting a title from the Bookvault system you should get a error notice pop-up that will indicate the ISBN can not be used in future (see below screenshot example). If you create a support ticket or email our team with the ISBN you wish to recover and we can arrange that with our IT team
