stbt control: Detect duplicated keyboard keys in the keymap file

If a user assigns multiple remote control keys to a keyboard key in the
keymap file, like

    o   OPEN
    o   OK

`stbt control` silently ignores this configuration issue and only
uses the mapping that is specified later in the keymap file.

Rase `stbt.ConfigurationError` if a keyboard key is duplicated.
Merge pull request #123 from szemate/stbt-control-to-detect-duplicated-key

stbt control: Detect duplicated keyboard keys in the keymap file