Add warning for multiple same checks per test

[martinhoyer]

Pull Request Checklist

• implement the feature

[happz]

@martinhoyer what about making it a linter check? In the normalization callback, the context it can provide is much smaller: yes, it reports multiple checks of the same how, but for which test? The callback does not see its "parent" object, therefore it cannot share this info. A linter check on test-level could.

[martinhoyer]

@martinhoyer what about making it a linter check? In the normalization callback, the context it can provide is much smaller: yes, it reports multiple checks of the same how, but for which test? The callback does not see its "parent" object, therefore it cannot share this info. A linter check on test-level could.

Make sense, will look into it. Thanks!