Do not quote already quoted identifiers in JdbcStatement.enquoteIdentifier()

h2/src/main/org/h2/jdbc/JdbcStatement.java

@@ -1383,18 +1383,36 @@ public class JdbcStatement extends TraceObject implements Statement, JdbcStateme
 
     /**
      * @param identifier
-     *            identifier to quote if required
+     *            identifier to quote if required, may be quoted or unquoted
      * @param alwaysQuote
      *            if {@code true} identifier will be quoted unconditionally
-     * @return specified identifier quoted if required or explicitly requested
+     * @return specified identifier quoted if required, explicitly requested, or
+     *         if it was already quoted
+     * @throws SQLException
+     *             if identifier is not a valid identifier
      */
     @Override
     public String enquoteIdentifier(String identifier, boolean alwaysQuote) throws SQLException {
-        if (alwaysQuote || !isSimpleIdentifier(identifier)) {
-            return StringUtils.quoteIdentifier(identifier);
+        if (isSimpleIdentifier(identifier)) {
+            return alwaysQuote ? '"' + identifier + '"': identifier;
+        }
+        int length = identifier.length();
+        if (length > 0 && identifier.charAt(0) == '"') {
+            boolean quoted = true;
+            for (int i = 1; i < length; i++) {
+                if (identifier.charAt(i) == '"') {
+                    quoted = !quoted;
+                } else if (!quoted) {
+                    throw new SQLException();
+                }
+            }
+            if (quoted) {
+                throw new SQLException();
             }
             return identifier;
         }
+        return StringUtils.quoteIdentifier(identifier);
+    }
 
     /**
      * @param identifier

h2/src/test/org/h2/test/jdbc/TestStatement.java

@@ -484,6 +484,15 @@ public class TestStatement extends TestDb {
         assertEquals("\"Test\"", stat.enquoteIdentifier("Test", false));
         assertEquals("\"test\"", stat.enquoteIdentifier("test", false));
         assertEquals("\"TODAY\"", stat.enquoteIdentifier("TODAY", false));
+        assertEquals("\"Test\"", stat.enquoteIdentifier("\"Test\"", false));
+        assertEquals("\"Test\"", stat.enquoteIdentifier("\"Test\"", true));
+        assertEquals("\"\"\"Test\"", stat.enquoteIdentifier("\"\"\"Test\"", true));
+        try {
+            stat.enquoteIdentifier("\"Test", true);
+            fail();
+        } catch (SQLException ex) {
+            // OK
+        }
         // Other lower case characters don't have upper case mappings
         assertEquals("\u02B0", stat.enquoteIdentifier("\u02B0", false));