Turning the creator wizard off (as suggested by Kristiaan Van den Eynde in the Drupal slack channel) seems to be a workaround for the newly created groups. A new node added to the new group shows up only once on the /group/3/nodes view (and on the /admin/nodes view as well) and there is only one admin member on the /group/3/members view with Group creator role.
The existing ones still have the admin account displayed four times on the members view (twice without role and twice with Group creator role). The root cause of this is the same - the groups were previously created with the Group creator must complete their membership option selected.
By deleting one of the admin users (e.g. the one added with id: 2 to the group during its creation), there remains only one admin user in the group with Group creator role. It seems that this might still need a fix considering the existing data.