Do the field names for custom fields need to be unique?

When specifying custom fields the docs state that for the unique key 'This is not often used but it can be used by 3rd party extensions or developers to retrieve a value of the custom field.'

Does this mean that if you had for example two profiles A and B both with a field "Home telephone" and a developer wanted to access this custom field in profile A that the key should be unique for each field (e.g. 'a_home_tel', 'b_home_tel')? Surely the field would be identified by both the profile ID and the field?

I ask because I just want to make sure I set up the fields correctly.