Allows single- and double-dashed args with the same name

[natecook1000]

Fixes #231. Note that this changes the error text slightly, to include the single/double-dash prefix.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test