Decide what needs to happen if a "from" value appears multiple times in the mapping(s)

[ericcornelissen]

Currently, the behaviour of the program is undefined if a "from" value appears in the mapping file(s) multiple times. This should be improved:

• Add warning log if it is detected
• Use the first or last value and document the decision

[ericcornelissen]

Referencing #91

[ericcornelissen]

As per #91, the last mapping that is defined will be used. This behaviour has yet to be documented.

As for the warning log, I think it is most appropriate to only inform the user about this in --verbose mode.