Disallow function redefinition (E0102).

### Motivation:
Redefined functions can hamper code readability and can lead to subtle errors. When intentional, they are an indicator of bad design. Let's rid ourselves of them.

### Changelog:
* Enable `pylint` redefinition check: `E0102`.
* Fix some present function redefinitions via renaming.
* Silence redefinition warnings in `behave` tests.