A Holyoke police sergeant has been suspended for the second time in a little more than two years for misplacing a department gun. Chief James Neiswanger said Wednesday that Sergeant John Hart was suspended 10 days without pay for leaving his gun in a restroom at the Holyoke Mall at Ingleside in November. The weapon was recovered. The Republican reported that Neiswanger said in an e-mail that an investigation found that Hart ‘‘violated several rules and regulations.’’ Hart was suspended for five days without pay in December 2011 for losing a department issued sniper rifle in September of that year. The rifle was recovered months later.