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The plumber said it was because it was new,
Your plumber's blowing smoke up your skirt. New faucets, whether it has a washer, are supposed to work upon installation. Period! Having said that I believe that some crud got in the valve body and is preventing it from completely shutting off. Not knowing the brand and model of your faucet I can't get specific but I would shut the water off and open up the valve. Check and remove any trash that you may find. This next step's important! Now, turn the water back on for a minute to flush out the supplies and the open valvebody. Reassemble and test. If that don't get it then demand the faucet be replaced under warranty at no cost to you. Good luck and keep me informed, Tom