BUG: Duplicate key causes undefined behaviour, user not warned

[corneliusroemer]

When trying to figure out what happens in #30 I discovered that when there are rows with duplicated keys, the behaviour is odd.

It would be good to test for duplicate keys and raise an error or warning (maybe allowing user to pick how to deal with this).

What shouldn't happen is that incomplete rows cause some sort of undefined behaviour as here:

printf "a,b,c,d\n1,2,3,4\n1,2,3\n3,2,3,4" >a.csv
printf "a,b,c,d\n1,2,3,4\n1,2,3,4\n3,2,3,4" >b.csv
csv-diff b.csv a.csv --key a

The output makes no sense:

1 column removed

1 column removed

  d