Auto merge of #2165 - psyomn:fix-2125-dup-bins, r=alexcrichton

Alex, thank you for directing me to the right way.

I did some changes, and made it bail with the following message if duplicate binaries are detected:

````nocode
failed to parse manifest at `/home/psyomn/programming/rust/fork/badproj/Cargo.toml`

Caused by:
  rejected binary name e: only unique binary names are accepted
````

I also added a test to check for this! Let me know if anything needs fixing.

ref #2125