Fix #556 Remove duplicate error messages from clang++

autoload/ale/handlers/gcc.vim

@@ -99,12 +99,17 @@ function! ale#handlers#gcc#HandleGCCFormat(buffer, lines) abort
                 continue
             endif
 
-            call add(l:output, {
+            let l:obj = {
             \   'lnum': l:match[2] + 0,
             \   'col': l:match[3] + 0,
             \   'type': l:match[4] =~# 'error' ? 'E' : 'W',
             \   'text': s:RemoveUnicodeQuotes(l:match[5]),
-            \})
+            \}
+
+            " clang++ and some other tools can output duplicated errors.
+            if empty(l:output) || l:output[-1] != l:obj
+                call add(l:output, l:obj)
+            endif
         endif
     endfor
 

test/handler/test_gcc_handler.vader

@@ -94,3 +94,29 @@ Execute(The GCC handler shouldn't complain about #pragma once for headers):
   \ ale#handlers#gcc#HandleGCCFormat(347, [
   \   '<stdin>:1:1: warning: #pragma once in main file [enabled by default]',
   \ ])
+
+Execute(The GCC handler should eliminate duplicated clang errors):
+  AssertEqual
+  \ [
+  \   {'lnum': 2, 'col': 10, 'type': 'E', 'text': '''a.h'' file not found'},
+  \   {'lnum': 4, 'col': 10, 'type': 'E', 'text': 'empty filename'},
+  \ ],
+  \ ale#handlers#gcc#HandleGCCFormat(347, [
+  \ '<stdin>:2:10: fatal error: ''a.h'' file not found',
+  \ '#include "a.h"',
+  \ '         ^~~~~',
+  \ '',
+  \ '<stdin>:2:10: fatal error: ''a.h'' file not found',
+  \ '#include "a.h"',
+  \ '         ^~~~~',
+  \ '',
+  \ '<stdin>:4:10: error: empty filename',
+  \ '',
+  \ '<stdin>:4:10: error: empty filename',
+  \ '#include ""',
+  \ '         ^',
+  \ '',
+  \ '<stdin>:4:10: error: empty filename',
+  \ '#include ""',
+  \ '         ^',
+  \ ])