Don't allow top level fields with the same name

[shcheklein]

Motivated by this: #510 (comment)

I'm not sure it should be allowed at all - to have a nested column meta and a regular field meta at the same time. Would it be a deal breaker for the scenario if we put a limitation like that @tibor-mach ?

caveats:

• if it's not too complicated to implement - ideally if it can be isolated in a single place)
• let's do a bit of research - e.g. does pandas allow for this? if yes, we can get back to the original ticket and see how we should clarify the semantics of operations like select in this case Bug: hierarchical columns and the .select method #510 (comment)