Bug #4742 getTableTypes computes wrong result

Description:
calling databaseMetaData.getTableTypes() returns a ResultSet, the first two columns representing a mapping from mysql column type names to java.sql.Types constants.

the first column has a unique constraint (virtually), as it represents the mapping key. but the type name "DOUBLE" gets mapped twice:

- DOUBLE --> 6
- DOUBLE --> 8

How to repeat:
    Connection connection = dataSource.getConnection();
    DatabaseMetaData metaData = connection.getMetaData();
    System.out.println("Driver Version: " + metaData.getDriverVersion());

    System.out.println("Type Mappings:");
    ResultSet rs = metaData.getTypeInfo();
    while (rs.next())
    {
      String name = rs.getString(1);
      int jdbcCode = rs.getInt(2);

      System.out.println("- " + name + " --> " + jdbcCode);
    }

I was able to verify this using 3.0.14 and 3.1.3-beta on linux connecting to 4.0.20 and 4.1.3-beta.

- DOUBLE --> 6
- FLOAT --> 7
- DOUBLE --> 8
- DOUBLE PRECISION --> 8

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