What is a double stitched seam?

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This question is open to interpretation.

Knits and spandex sportwear/swimwear is finished on a serger or overlock machine.

Two neat stitch lines appear on the 'right' side of the garment. When you reverse the fabric you see that these stitch lines are connected (overlocked).

Double stitched may refer to two rows of stitching you typically see on Denim jeans. This is a flat felled seam- no loose ends or seams to overcast to prevent the denim from unravelling.

A double stitched seam may be straight stitching on a seam and then stitching another row alongside it.