All rights reserved. All load bearing seams require double stitching for strength and durability. Fortunately, all manufacturers do this.
Some companies go the extra length to triple and quadruple stitch the seams.
For non-load bearing seams such as the partial ribs used on gliders, a single line of stitching is appropriate. There is no structural need to double stitch the seam and add unnecessary costs to produce the glider. Most early paraglider manufacturers used one row of straight stitching and another row of zigzag stitching side by side. A zigzag stitch has more give and is less subject to breakage. Thread consumption is higher since the stitches lie diagonally across the fabric.