The Double Lineman's Knot is great for an emergency bosun's chair or a sling for a ladder, or a low-chafe shackle attachment, or a Trucker's Hitch.
To Tie a Double Lineman's Loop:
Start with a loosely made Single Lineman's Loop. Hold the knot and standing parts with one hand. With your other hand, open up the loop and fold it down over the knot.
Pull up on the 2 bights that lead directly to the loop.
Draw the 2 bights up to form 2 loops. The original loop will draw up around the 2 standing parts. If the bights do no draw out readily, try holding the knot itself with one hand while pulling up on the bights with the other.