After the ISB standard signature has been applied to your account, you need to delete or turn off any signatures you may have created:
PC Outlook:
MAC Outlook > Preferences> Signatures:
Web: In Outlook, click the Cog top right> find “Your app settings”, Click ‘Mail’, then on the left side under “Layout”, Click ‘Email signature’, ensure both boxes are unchecked:
You should also clear any signatures from your mobile devices.
This will ensure that your signatures don’t double up.