* CRB index virtually unchanged for 2 day * Brent crude ends up 6 cents after doubts on oil demand * Metals in quite trade ahead of G20 meeting * Natgas jumps 2 pct on forecasts for cold U.S. weather * Grains markets see little price change By Barani Krishnan NEW YORK, Feb 13 (Reuters) - Most commodities were trapped in range-bound trading for a second straight day on Wednesday, with oil hampered by doubts over demand and metals affected by caution ahead of a meeting of