The Morocco full-back scored after five minutes and added his second five minutes into added time to put PSG on 21 points from seven games, three days after the capital side beat Lyon by the same scoreline thanks to another winning goal in the dying seconds.
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Metz, who finished with 10 men after Dylan Bronn picked up a second yellow card in the last minute, have three points from seven games.
In Metz, PSG were without Lionel Messi after the Argentine forward was ruled out with a bruised knee, but Hakimi stepped up.
Five minutes later, Hakimi curled a superb shot into the net after being set up by Neymar, giving Mauricio Pochettino's side all three points after another below-par performance.