diff --git a/lib/lrama/new_parser.rb b/lib/lrama/new_parser.rb
index 418e5cd3..08b60a7f 100644
--- a/lib/lrama/new_parser.rb
+++ b/lib/lrama/new_parser.rb
@@ -32,7 +32,7 @@ def next_token
 end
 
 def prologue(s_value)
-  if @header_path
+  if @header_path && !s_value.match?(/^\s*#include\s+"#{@header_path}"/)
     "\n#include \"#{@header_path}\"\n" + s_value
   else
     s_value
diff --git a/parser.y b/parser.y
index 9a0a2ee8..9775c3ce 100644
--- a/parser.y
+++ b/parser.y
@@ -385,7 +385,7 @@ def next_token
 end
 
 def prologue(s_value)
-  if @header_path
+  if @header_path && !s_value.match?(/^\s*#include\s+"#{@header_path}"/)
     "\n#include \"#{@header_path}\"\n" + s_value
   else
     s_value