Fix more UTs

Author: julian-elastic

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -346,7 +346,6 @@ static TransportVersion def(int id) {
     public static final TransportVersion ML_INFERENCE_LLAMA_ADDED = def(9_125_0_00);
     public static final TransportVersion ESQL_LOOKUP_JOIN_ON_MANY_FIELDS = def(9_126_0_00);
 
-
     /*
      * STOP! READ THIS FIRST! No, really,
      *        ____ _____ ___  ____  _        ____  _____    _    ____    _____ _   _ ___ ____    _____ ___ ____  ____ _____ _

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/LogicalPlanBuilder.java

@@ -634,6 +634,21 @@ public PlanFactory visitJoinCommand(EsqlBaseParser.JoinCommandContext ctx) {
             }
         }
 
+        var matchFieldsCount = joinFields.size();
+        if (matchFieldsCount > 1) {
+            Set<String> matchFieldNames = new LinkedHashSet<>();
+            for (Attribute field : joinFields) {
+                if (matchFieldNames.add(field.name()) == false) {
+                    throw new ParsingException(
+                        field.source(),
+                        "JOIN ON clause does not support multiple fields with the same name, found multiple instances of [{}]",
+                        field.name()
+                    );
+                }
+
+            }
+        }
+
         return p -> {
             boolean hasRemotes = p.anyMatch(node -> {
                 if (node instanceof UnresolvedRelation r) {

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/analysis/ParsingTests.java

@@ -135,26 +135,18 @@ public void testJoinOnConstant() {
         );
     }
 
-    public void testJoinOnMultipleFields() {
-        assumeTrue("LOOKUP JOIN available as snapshot only", EsqlCapabilities.Cap.JOIN_LOOKUP_V12.isEnabled());
-        assertEquals(
-            "1:35: JOIN ON clause only supports one field at the moment, found [2]",
-            error("row languages = 1, gender = \"f\" | lookup join test on gender, languages")
-        );
-    }
-
     public void testJoinTwiceOnTheSameField() {
         assumeTrue("LOOKUP JOIN available as snapshot only", EsqlCapabilities.Cap.JOIN_LOOKUP_V12.isEnabled());
         assertEquals(
-            "1:35: JOIN ON clause only supports one field at the moment, found [2]",
+            "1:66: JOIN ON clause does not support multiple fields with the same name, found multiple instances of [languages]",
             error("row languages = 1, gender = \"f\" | lookup join test on languages, languages")
         );
     }
 
     public void testJoinTwiceOnTheSameField_TwoLookups() {
         assumeTrue("LOOKUP JOIN available as snapshot only", EsqlCapabilities.Cap.JOIN_LOOKUP_V12.isEnabled());
         assertEquals(
-            "1:80: JOIN ON clause only supports one field at the moment, found [2]",
+            "1:108: JOIN ON clause does not support multiple fields with the same name, found multiple instances of [gender]",
             error("row languages = 1, gender = \"f\" | lookup join test on languages | eval x = 1 | lookup join test on gender, gender")
         );
     }