Rule Request: Duplicate Imports

[jpsim]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

Duplicate imports are unnecessary.

2. Provide several examples of what would and wouldn't trigger violations.

// Should Trigger
import Foundation
import Dispatch
import Foundation

// Should Trigger
import Foundation
import Foundation.NSString

// Should Not Trigger
Import Foundation.NSString
Import Foundation.NSRange

3. Should the rule be configurable, if so what parameters should be configurable?

Shouldn't be configurable. Either there are duplicates or not.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a
   rule as opt-in.

Enabled by default.

@marcelofabri removing this duplicate import motivated me to write this rule request: https://github.com/realm/SwiftLint/pull/1872/files#diff-71e28580a5fd3b8b3f701a7c8ca9f6a9

There are gotchas around conditional imports, so I suggest we just not count imports in a conditional compilation expression.

[marcelofabri]

Nice, I thought about filling an issue requesting this rule but I completely forgot later.

[marcelofabri]

Implemented in #2004