diff --git a/parse.y b/parse.y
index 21cb6e4fa3..8b5e1d5ad7 100644
--- a/parse.y
+++ b/parse.y
@@ -250,6 +250,7 @@ struct parser_params {
     int heredoc_line_indent;
     char *tokenbuf;
     struct local_vars *lvtbl;
+    st_table *pvtbl;
     int line_count;
     int ruby_sourceline;	/* current line no. */
     const char *ruby_sourcefile; /* current source file */
@@ -496,6 +497,8 @@ static NODE *heredoc_dedent(struct parser_params*,NODE*);
 
 static void check_literal_when(struct parser_params *p, NODE *args, const YYLTYPE *loc);
 
+static void error_duplicate_pattern_variable(struct parser_params *p, ID id, const YYLTYPE *loc);
+
 #define get_id(id) (id)
 #define get_value(val) (val)
 #define get_num(num) (num)
@@ -974,6 +977,7 @@ static int looking_at_eol_p(struct parser_params *p);
     NODE *node;
     ID id;
     int num;
+    st_table *tbl;
     const struct vtable *vars;
     struct rb_strterm_struct *strterm;
 }
@@ -1561,14 +1565,22 @@ expr		: command_call
 			$<num>$ = p->in_kwarg;
 			p->in_kwarg = 1;
 		    }
+		    {
+			$<tbl>$ = p->pvtbl;
+			p->pvtbl = st_init_numtable();
+		    }
 		  p_top_expr_body
+		    {
+			st_free_table(p->pvtbl);
+			p->pvtbl = $<tbl>4;
+		    }
 		    {
 			p->in_kwarg = !!$<num>3;
 		    /*%%%*/
-			$$ = NEW_CASE3($1, NEW_IN($4, NEW_TRUE(&@4), NEW_FALSE(&@4), &@4), &@$);
+			$$ = NEW_CASE3($1, NEW_IN($5, NEW_TRUE(&@5), NEW_FALSE(&@5), &@5), &@$);
 			rb_warn0L(nd_line($$), "Pattern matching is experimental, and the behavior may change in future versions of Ruby!");
 		    /*% %*/
-		    /*% ripper: case!($1, in!($4, Qnil, Qnil)) %*/
+		    /*% ripper: case!($1, in!($5, Qnil, Qnil)) %*/
 		    }
 		| arg %prec tLBRACE_ARG
 		;
@@ -3800,7 +3812,15 @@ p_case_body	: keyword_in
 			$<num>$ = p->in_kwarg;
 			p->in_kwarg = 1;
 		    }
+		    {
+			$<tbl>$ = p->pvtbl;
+			p->pvtbl = st_init_numtable();
+		    }
 		  p_top_expr then
+		    {
+			st_free_table(p->pvtbl);
+			p->pvtbl = $<tbl>3;
+		    }
 		    {
 			p->in_kwarg = !!$<num>2;
 		    }
@@ -3808,9 +3828,9 @@ p_case_body	: keyword_in
 		  p_cases
 		    {
 		    /*%%%*/
-			$$ = NEW_IN($3, $6, $7, &@$);
+			$$ = NEW_IN($4, $8, $9, &@$);
 		    /*% %*/
-		    /*% ripper: in!($3, $6, escape_Qundef($7)) %*/
+		    /*% ripper: in!($4, $8, escape_Qundef($9)) %*/
 		    }
 		;
 
@@ -4089,6 +4109,7 @@ p_kw		: tLABEL p_expr
 			    yyerror1(&@1, "key must be valid as local variables");
 			}
 		    /*%%%*/
+			error_duplicate_pattern_variable(p, $1, &@1);
 			$$ = list_append(p, NEW_LIST(NEW_LIT(ID2SYM($1), &@$), &@$), assignable(p, $1, 0, &@$));
 		    /*% %*/
 		    /*% ripper: rb_ary_new_from_args(1, rb_ary_new_from_args(2, get_value($1), Qnil)) %*/
@@ -4119,6 +4140,7 @@ p_kw		: tLABEL p_expr
 			    if (!is_local_id(id)) {
 				yyerror1(&loc, "key must be valid as local variables");
 			    }
+			    error_duplicate_pattern_variable(p, id, &loc);
 			    $$ = list_append(p, NEW_LIST(node, &loc), assignable(p, id, 0, &@$));
 			}
 			else {
@@ -4249,6 +4271,7 @@ p_primitive	: literal
 p_variable	: tIDENTIFIER
 		    {
 		    /*%%%*/
+			error_duplicate_pattern_variable(p, $1, &@1);
 			$$ = assignable(p, $1, 0, &@$);
 		    /*% %*/
 		    /*% ripper: assignable(p, var_field(p, $1)) %*/
@@ -11529,6 +11552,20 @@ new_hash(struct parser_params *p, NODE *hash, const YYLTYPE *loc)
     return NEW_HASH(hash, loc);
 }
 
+static void
+error_duplicate_pattern_variable(struct parser_params *p, ID id, const YYLTYPE *loc)
+{
+    if (is_private_local_id(id)) {
+	return;
+    }
+    if (st_is_member(p->pvtbl, id)) {
+	yyerror1(loc, "duplicated variable name");
+    }
+    else {
+	st_insert(p->pvtbl, (st_data_t)id, 0);
+    }
+}
+
 static void
 error_duplicate_keys(struct parser_params *p, NODE *hash)
 {
diff --git a/test/ruby/test_pattern_matching.rb b/test/ruby/test_pattern_matching.rb
index 5283d5ce87..0338f90a91 100644
--- a/test/ruby/test_pattern_matching.rb
+++ b/test/ruby/test_pattern_matching.rb
@@ -193,25 +193,25 @@ class TestPatternMatching < Test::Unit::TestCase
   def test_var_pattern
     # NODE_DASGN_CURR
     assert_block do
-      case [0, 1]
-      in a, a
-        a == 1
+      case 0
+      in a
+        a == 0
       end
     end
 
     # NODE_DASGN
     b = 0
     assert_block do
-      case [0, 1]
-      in b, b
+      case 1
+      in b
         b == 1
       end
     end
 
     # NODE_LASGN
-    case [0, 1]
-    in c, c
-      assert_equal(1, c)
+    case 0
+    in c
+      assert_equal(0, c)
     else
       flunk
     end
@@ -221,6 +221,57 @@ class TestPatternMatching < Test::Unit::TestCase
       in ^a
       end
     }, /no such local variable/)
+
+    assert_syntax_error(%q{
+      case 0
+      in a, a
+      end
+    }, /duplicated variable name/)
+
+    assert_block do
+      case [0, 1, 2, 3]
+      in _, _, _a, _a
+        true
+      end
+    end
+
+    assert_syntax_error(%q{
+      case 0
+      in a, {a:}
+      end
+    }, /duplicated variable name/)
+
+    assert_syntax_error(%q{
+      case 0
+      in a, {"a":}
+      end
+    }, /duplicated variable name/)
+
+    assert_block do
+      case [0, "1"]
+      in a, "#{case 1; in a; a; end}"
+        true
+      end
+    end
+
+    assert_syntax_error(%q{
+      case [0, "1"]
+      in a, "#{case 1; in a; a; end}", a
+      end
+    }, /duplicated variable name/)
+
+    assert_block do
+      case 0
+      in a
+        true
+      in a
+        flunk
+      end
+    end
+
+    assert_syntax_error(%q{
+      0 in [a, a]
+    }, /duplicated variable name/)
   end
 
   def test_literal_value_pattern