The double Daled is mentioned in the Pri Etz Chaim and the Mishnat Chassidim that the double Daled is to resemble a Mem Sofit (or Stuma).
This appears to be the practice of Rav Natan Adler and the Chasam Sofer (& his descendants and Talmidim).
Also practiced by the Sanzer Chassidim (the Double Daled is sometimes called the Sanz knot).
It also appears to be practiced by some Mekubalim of Bet El.
The source is an interesting statement in the Zohar (Pinchas): to tie/knot behind a Daled reverse/upside down which some interpret as a reverse daled behind a straight daled.
It appears (my own observations) not to be practiced by mainstream Ashkenazim, mainstream Sefardim, Chabad Lubavitch Chassidim and even some Ashkenazim that daven Nusach Sfard.