A rare, double-wide Gilded Age mansion by Prospect Park in Brooklyn is on the market for $16 million, Gimme Shelter can reveal. Built in 1887, the Park Slope mansion at 842 Carroll St. was designed by architect C.P.H. Gilbert.  It’s one of three in a row designed by Gilbert and part of the Park Slope Historic District — and the only one that has retained its single-family mansion status, said listing brokers Thomas Handschiegel and Mackenzie Kyle, of Platinum Forbes Global Properties. And, at 32 feet wide, it is one of Brooklyn’s widest townhouses — far wider than the average 15- to 20-foot-wide structures.  The double-wide Gilded Mansion at 842 Carroll St. Platinum Forbes Global Properties The home features exquisite details from the sky mural to the original woodwork. Platinum Forbes Global Properties The formal dining room. Platinum Forbes Global Properties One of the home’s 10 woodburning fireplaces.…