Line doc "already defined" error

[huacnlee]

If we define line comment has multiple same contents, then will get compile error.

/// Matches dar
/// 
/// Match dar description
/// 
dar = { "da" }

This line doc has 2 empty lines, then will error:

thread 'tests::test_generate_doc' panicked at 'grammar error

  --> 19:1
   |
19 | ///
   | ^--^
   |
   = rule ///  already defined', /Users/jason/github/pest/meta/src/lib.rs:42:9