You can display just one banner by searching in the ACP for "User Banners". Untick "Allow Banner Stacking". You can then change the banner styling priority in the user groups so that you can display the preferred banner when a user is in multiple user groups.
To use the same banner, create the same one in each user group and, if required, create custom user banner CSS:
https://xenforo.com/community/resources/creating-custom-user-group-banner-css.2000/