Duplicate keys in the Localicipe should result in an error

[LcTwisk]

Currently, if you by mistake define duplicate keys in the Localicipe, the previous entries will always be overwritten by the last key in the file.

The problem:
Let's say there's a Localicipe that looks like:

COLLECTION-A:
  Onboarding:
    Login:
      InputField: 
        Placeholder: 
          COPY:
            en: "Username"
      InputField: 
        Placeholder: 
          COPY:
            en: "E-mail"

The key Onboarding.Login.InputField.Placeholder is defined twice, as a result, the second entry will be overwritten by the first one. This behaviour might be confusing, entries that accidentally have duplicated keys will get lost.

Proposal
Improve the validation by throwing a proper error that indicates that duplicated entries are not allowed in the Localicipe.