stbt control: Detect duplicated keyboard keys in the keymap file
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.

LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester