Remove a duplicate function.

autoload/syntastic/c.vim

@@ -42,17 +42,6 @@ endfunction
 let s:default_includes = [ '.', '..', 'include', 'includes',
             \ '../include', '../includes' ]
 
-" uniquify the input list
-function! s:Unique(list)
-    let l = []
-    for elem in a:list
-        if index(l, elem) == -1
-            let l = add(l, elem)
-        endif
-    endfor
-    return l
-endfunction
-
 " convenience function to determine the 'null device' parameter
 " based on the current operating system
 function! syntastic#c#GetNullDevice()
@@ -78,7 +67,7 @@ function! syntastic#c#GetIncludeDirs(filetype)
         call extend(include_dirs, g:syntastic_{a:filetype}_include_dirs)
     endif
 
-    return join(map(s:Unique(include_dirs), '"-I" . v:val'), ' ')
+    return join(map(syntastic#util#unique(include_dirs), '"-I" . v:val'), ' ')
 endfunction
 
 " read additional compiler flags from the given configuration file

autoload/syntastic/util.vim

@@ -129,13 +129,13 @@ function! syntastic#util#compareErrorItems(a, b)
     endif
 endfunction
 
-" List of buffers referenced by the location list
+" Returns unique elements in a list
 function! syntastic#util#unique(list)
     let seen = {}
     for e in a:list
         let seen[e] = 1
     endfor
-    return keys(seen)
+    return copy(keys(seen))
 endfunction
 
 function! syntastic#util#debug(msg)