Thanks for the comments Derrick Wong and Graham Hudgins. I've done a few tests and added some pictures. Hopefully that will help!
I'm not convinced groups are being created (at least not group groups - a 'Site' may be considered a type of 'group'???). When I create a Site, there is a dialogue along the bottom of the page that certainly indicates a group is being created, however when i search for the group afterwards, there is nothing to be found.
Also I can create a Site with the same name as an existing group. And a Site with the same name as an existing Site! When you do this, -1, -2, etc. is appended to the URL name of the Site, but not the name you see in your Content. So an existing group is not the issue.
For example, if I already have a group called 'Bananas', then create a Site called 'Bananas' (no errors returned at that name), the Site URL will become //bananas-1. If i create another Site called 'Bananas' (no errors returned at that name either), the Site URL will become //bananas-2. So I can have a bunch of Bananas.
The HUB Site i am trying to create is 'Pipeline Surveillance'.
I get an issue as with 'Pipelin', 'Pipeline' and 'Pipeline ', and again at 'Pipeline Surveil', and 'Pipeline Surveill'. 'Pipeline Surveillance' is not ok also.
Though 'Pipeline Surveilla', 'Pipeline Surveillan', and 'Pipeline Surveillanc' are ok!
These combinations causing issues are weird in themselves! 'Pipeline Surveillan' is not ok but 'Pipeline Surveillanc' is? What's that all about?!!
I would be interested to see if either of you see this issue?
Your comment about a registered name is interesting Derrick Wong... Could this be a problem even thought the URL for a Site contains your ArcGIS Online Organisation name?
When you mention group names Graham Hudgins, the name is just an alias isn't it? The group is identified by an ID (e.g. da99b6b0a4e94bf59726278b45e8cb4a). Is the alias used over the ID behind the scenes anywhere?
Thanks again.