Thank you very much.
email = models.EmailField(unique=True, error_messages={'unique':"This email has already been registered."})
this worked very well now.
If you want to customise error_messages like invalided, do it in forms.ModelForm
email = forms.EmailField(error_messages={'invalid': 'Your email address is incorrect'})
But unique message should be customised in model field, as ben mentioned
email = models.EmailField(unique=True, error_messages={'unique':"This email has already been registered."})