President Reagan has been wearing a duplicate of the cashmere and wool navy chalk-stripe suit he had on when he was shot last year. Frank Mariani, the California tailor who with his father has been responsible for the president's suits for more than 40 years, says Nancy Reagan requested an exact copy of the suit. "It was one of the president's favorites," she told Mariani.

On a trip to California last summer, Mrs. Reagan checked a swatch of the fabric for the new suit and gave Mariani the go-ahead. Apparently the president liked it -- "He's already worn it on television," says Mariani.

Mariani made the inaugural outfit for the president and at least 10 suits since then, he says. Mrs. Reagan approves most of the colors and patterns, which are never very extreme and often in a luxurious "Super-100" wool and cashmere fabric. Mariani says the president pays up to about $1,200 for each custom-tailored suit.

If he bought clothes off the rack, the president would be a perfect 42 long, according to Mariani, who says the president's current preference is for grays and blues. "No man has a complete wardrobe without a few brown suits," says Mariani. "Brown is a hard color for older men to wear because it may make them sallow and wash them out. But the president has very good coloring."

The president's penchant for plaids comes not only from his California exposure, but also his Irish heritage. Just the same, Mariani calls the president one of his most conservative customers. His suits are single-breasted with a slightly suppressed waist, and all of his pants are pleated and have cuffs.

President Reagan made many visits to Mariani's Beverly Hills shop for fittings before his election. "We have his base measurement, but some fabrics have their own peculiarity that require different refinements in the fitting," says Mariani, who refers to the president and his wife as Ron and Nancy.

Like the first lady, the president wears his clothes for many years. "We cleaned and pressed a suit for him before the election that we originally made in 1963," says Mariani. In fact, Reagan sends many of his suits back to Mariani for cleaning and pressing.

The FBI has the original navy chalk-stripe suit that will be used as evidence in the trial of the man accused of shooting him. Then it will be returned to the president.