Not sure yet if it has to do with me deleting one template to try...
However, the duplicated templates have disappear after the update of XOA to 5.74.3!
Maybe a restart of the service would have achieved the same!?
At the end, it seems to come from XOA rather than the pool.

I suggest to remove and add the master from the server settings to see if this does not clean up something in the cache/DB on XOA...

I've never noticed before today, but indeed: all default templates appeared duplicated.
And deleting one instance seems to delete the second too!
Unfortunately, this is messing up Terraform plan with the xenorchestra provider:

module.loc0.xenorchestra_vm.vm0: Refreshing state... [id=<vm_uuid>]
Error: found `2` templates with query {Id: Uuid: NameLabel:Other install media PoolId:<pool_uuid> TemplateInfo:{Arch: Disks:[]}}. Templates must be uniquely named to use this data source

It seems like I have to re-install the whole pool, as new one do not seem to present this issue!