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.