Config file with duplicated keys passes validation

Setup:
1. config file with duplicate key/value pairs.
1. latest confcrypt version

Actual behavior:
* `confcrypt aws validate file.econf` outputs no validation erros.
* `confcrypt aws edit ...` removes duplicated keys.

Expected behavior:
* `confcrypt aws validate file.econf` reports duplicated keys.
* `confcrypt aws edit ...` validates file prior to editing.

Start by running `confcrypt aws validate file.econf` and `confcrypt aws edit ...` with a config containing duplicated keys. Trace the validation and edit entry points for how duplicate keys are parsed and handled. Done means validation reports duplicated keys and editing validates the file before removing or changing them.

Written by the indexing model from the issue text.