Avoid selection of duplicate columns.

Closes #1865

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/SqlGenerator.java

@@ -514,7 +514,7 @@ class SqlGenerator {
 
 		Table table = getTable();
 
-		List<Expression> columnExpressions = new ArrayList<>();
+		Set<Expression> columnExpressions = new LinkedHashSet<>();
 
 		List<Join> joinTables = new ArrayList<>();
 		for (PersistentPropertyPath<RelationalPersistentProperty> path : mappingContext

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/core/convert/SqlGeneratorUnitTests.java

@@ -43,6 +43,7 @@ import org.springframework.data.relational.core.dialect.SqlServerDialect;
 import org.springframework.data.relational.core.mapping.AggregatePath;
 import org.springframework.data.relational.core.mapping.Column;
 import org.springframework.data.relational.core.mapping.DefaultNamingStrategy;
+import org.springframework.data.relational.core.mapping.MappedCollection;
 import org.springframework.data.relational.core.mapping.RelationalMappingContext;
 import org.springframework.data.relational.core.mapping.RelationalPersistentEntity;
 import org.springframework.data.relational.core.mapping.RelationalPersistentProperty;
@@ -388,7 +389,8 @@ class SqlGeneratorUnitTests {
 	void findAllByPropertyWithKey() {
 
 		// this would get called when ListParent is th element type of a Map
-		String sql = sqlGenerator.getFindAllByProperty(BACKREF, new AggregatePath.ColumnInfo(unquoted("key-column"),unquoted("key-column")), false);
+		String sql = sqlGenerator.getFindAllByProperty(BACKREF,
+				new AggregatePath.ColumnInfo(unquoted("key-column"), unquoted("key-column")), false);
 
 		assertThat(sql).isEqualTo("SELECT dummy_entity.id1 AS id1, dummy_entity.x_name AS x_name, " //
 				+ "dummy_entity.x_other AS x_other, " //
@@ -926,6 +928,16 @@ class SqlGeneratorUnitTests {
 				"WHERE referenced_entity.parentId");
 	}
 
+	@Test // GH-1865
+	void mappingMapKeyToChildShouldNotResultInDuplicateColumn() {
+
+		SqlGenerator sqlGenerator = createSqlGenerator(Child.class);
+		String sql = sqlGenerator.getFindAllByProperty(Identifier.of(unquoted("parent"), 23, Parent.class),
+				context.getAggregatePath(getPath("children", Parent.class)).getTableInfo().qualifierColumnInfo(), false);
+
+		assertThat(sql).containsOnlyOnce("child.NICK_NAME AS NICK_NAME");
+	}
+
 	@Nullable
 	private SqlIdentifier getAlias(Object maybeAliased) {
 
@@ -1117,4 +1129,10 @@ class SqlGeneratorUnitTests {
 		@Id Long id;
 		IdNoIdChain idNoIdChain;
 	}
+
+	record Parent(@Id Long id, String name, @MappedCollection(keyColumn = "NICK_NAME") Map<String, Child> children) {
+	}
+
+	record Child(@Column("NICK_NAME") String nickName, String name) {
+	}
 }