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The Seagull Sierra, designed in 1980 by Tom Peghiny, was Seagull's only double-surface glider. I do not believe it was ever offered for sale, as Seagull went defunct before the glider was HGMA certified. I remember a few prototypes flying in 1980; these were reported to have good performance but poor handling characteristics, possibly due to a very tight sail and insufficiently flexible airframe.