cabal should warn if dependency is specified multiple times

The buildbox-2.1.2.2 package has the following Build-Depends:

```
Build-Depends:
base >= 4.4 && < 4.7,
containers >= 0.4 && < 0.6,
time >= 1.2 && < 1.5,
directory >= 1.1 && < 1.3,
bytestring >= 0.9 && < 0.11,
mtl == 2.1.*,
old-locale == 1.0.*,
process == 1.1.*,
random == 1.0.*,
pretty == 1.1.*,
bytestring == 0.10.*,
stm == 2.4.*
```

Note that bytestring is defined twice. cabal should detect and reject multiple definitions of the same dependency.

**Editorial by @ezyang.** Note that, with the presence of conditionals, a particular dependency can easily show up multiple times in a Cabal file. So the request here is more for a "lint" type check for when the duplication cannot possibly help.