Double feather stitch is a wider, slightly more elaborate stitch than feather stitch: it has three fly stitches in a row before it changes direction, rather than two.

If four fly stitches are stitched before the change of direction, the stitch is sometimes known as triple feather stitch.

The descriptive alternative name of briar stitch (a thorny plant) is also used to refer to feather stitch although it seems more apt for the more numerous thorns in his version.

Examples of the use of double feather stitch, exist from the 19th century: the Victoria and Albert Museum, London has a sampler from 1887 which clearly shows a line of double feather stitch.  It also features on a crazy quilt from the previous year held by the Art Institute of Chicago where is it used to join quilting blocks.

For more information see the entry for feather stitch.