Update CHANGELOG and doc comment

CHANGELOG.md

@@ -31,6 +31,10 @@
 
 ## [0.6.8]
 
+### Added
+
+ - Detects duplicate filenames.
+
 ### Fixed
 
  - `deep_copy_file` could set incorrect Unix permissions.
@@ -40,4 +44,4 @@
  
 ### Changed
 
- - Improved performance of `deep_copy_file`: it no longer searches for the filename twice.
+ - Improved performance of `deep_copy_file` by using a HashMap and eliminating a redundant search.

src/write.rs

@@ -494,7 +494,8 @@ impl<W: Write + Seek> ZipWriter<W> {
         Ok(())
     }
 
-    /// Create a file in the archive and start writing its' contents.
+    /// Create a file in the archive and start writing its' contents. The file must not have the
+    /// same name as a file already in the archive.
     ///
     /// The data should be written using the [`Write`] implementation on this [`ZipWriter`]
     pub fn start_file<S>(&mut self, name: S, mut options: FileOptions) -> ZipResult<()>