Dedup overrides by name

Overrides are only queried by name, and it's possible for multiple to show up,
and just pick the first one.

Closes #461
auto merge of #474 : alexcrichton/cargo/issue-461, r=wycats

Overrides are only queried by name, and it's possible for multiple to show up,
and just pick the first one.

Closes #461 

r? @wycats -- I'm not convinced this is correct