hotfix for duplicates in JOIN ON #4271

dbms/src/Interpreters/Join.cpp

@@ -469,9 +469,15 @@ bool Join::insertFromBlock(const Block & block)
     }
     else
     {
+        NameSet erased; /// HOTFIX: there could be duplicates in JOIN ON section
+
         /// Remove the key columns from stored_block, as they are not needed.
         for (const auto & name : key_names_right)
-            stored_block->erase(stored_block->getPositionByName(name));
+        {
+            if (!erased.count(name))
+                stored_block->erase(stored_block->getPositionByName(name));
+            erased.insert(name);
+        }
     }
 
     size_t size = stored_block->columns();

dbms/tests/queries/0_stateless/00818_join_bug_4271.reference

@@ -0,0 +1,9 @@
+1	1	a	1	1	a
+2	2	b	\N	\N	\N
+1	1	a	1	1	a
+2	2	b	\N	\N	\N
+1	1	a	1	1	a
+1	1	a	1	1	a
+2	2	b	\N	\N	\N
+1	1	a	1	1	a
+2	2	b	\N	\N	\N

dbms/tests/queries/0_stateless/00818_join_bug_4271.sql

@@ -0,0 +1,19 @@
+use test;
+
+drop table if exists t;
+drop table if exists s;
+
+create table t(a Nullable(Int64), b Nullable(Int64), c Nullable(String)) engine = Memory;
+create table s(a Nullable(Int64), b Nullable(Int64), c Nullable(String)) engine = Memory;
+
+insert into t values(1,1,'a'), (2,2,'b');
+insert into s values(1,1,'a');
+
+select * from t left join s on t.a = s.a;
+select * from t left join s on t.a = s.a and t.a = s.b;
+select * from t left join s on t.a = s.a where s.a = 1;
+select * from t left join s on t.a = s.a and t.a = s.a;
+select * from t left join s on t.a = s.a and t.b = s.a;
+
+drop table t;
+drop table s;