Adding a default + alias repeats the aliases

When adding both a default and an alias, and then printing the help, it repeats the argument.

### Example

```javascript
yargs('')
    .alias('f', 'foo')
    .default('f', 5)
    .demand(1)
    .argv
```

outputs

```
Options:
      -f, --foo  [default: 5]
      --foo      [default: 5]
Not enough non-option arguments: got 0, need at least 1
```