variable uniqueness group_vars

I have the following

hosts file https://gist.github.com/anonymous/6c3e001ad76208b21296
playbook https://gist.github.com/anonymous/012130d10b10838dc03b

My server groups align with roles and I have some overlapping concepts, like a server_name. When I run my playbook, the value for server_name defined in the bottle group is being used in the wordpress group.

Is it possible to have variable names overlap when defined in the hosts file, like I have done?

My next question would be whether I can have overlapping names when using the group_vars folder, as here http://docs.ansible.com/ansible/intro_inventory.html#groups-of-groups-and-group-variables

Thanks,
Daniel