From 4881fff318adbcf2abfa2e2c07a5cc08e0585b14 Mon Sep 17 00:00:00 2001
From: Daksh Shah <daksh17336@iiitd.ac.in>
Date: Wed, 23 Dec 2020 14:38:06 +0530
Subject: [PATCH] Remove duplicate productions in CNF for CFGs (#2435)

Co-authored-by: Steven Bird <stevenbird1@gmail.com>
---
 AUTHORS.md      | 1 +
 nltk/grammar.py | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/AUTHORS.md b/AUTHORS.md
index aa40a24d4e..20180034e2 100644
--- a/AUTHORS.md
+++ b/AUTHORS.md
@@ -263,6 +263,7 @@
 - Nicolas Darr <https://github.com/ndarr>
 - Hervé Nicol <https://github.com/hervenicol>
 - Alexandre H. T. Dias <https://github.com/alexandredias3d>
+- Daksh Shah <https://github.com/Daksh>
 - Jacob Weightman <https://github.com/jacobdweightman>
 - Bonifacio de Oliveira <https://github.com/Bonifacio2>
 - Armins Bagrats Stepanjans <https://github.com/ab-10>
diff --git a/nltk/grammar.py b/nltk/grammar.py
index fe04c01b97..97e6741ef4 100644
--- a/nltk/grammar.py
+++ b/nltk/grammar.py
@@ -770,7 +770,8 @@ def chomsky_normal_form(self, new_token_padding="@$@", flexible=False):
         if flexible:
             return step2
         step3 = CFG.remove_unitary_rules(step2)
-        return step3
+        step4 = CFG(step3.start(),list(set(step3.productions())))
+        return step4
 
     @classmethod
     def remove_unitary_rules(cls, grammar):