Combine soy sauce, vinegar, honey, garlic, ginger, and tarragon for marinade and immerse duck breasts. Marinate 24 hours in the refrigerator.
Prepare charcoal grill for indirect heat: Light 50 to 60 briquettes and let burn until covered with ash, 20 to 30 minutes, and very hot (you can hold your hand 1 to 2 inches above grill level only 1 to 2 seconds). Push equal portions of the coals to the sides and set a drip pan filled 1/2 in. up sides with water in the cleared area. Grill duck skin side down over cleared area (close lid and keep vent holes open), basting with marinade, 6 minutes on each side. Move breasts over direct heat and cook, skin side down, until skin is well browned and crisp and meat is medium-rare (cut to check), 5 to 10 minutes. A small spray bottle filled with water is handy for taming any flames.
Place duck skin side up on an ovenproof platter and cover meat generously with marmalade. Broil in oven until marmalade bubbles and thickens. Garnish with parsley and serve.
Note: Nutritional analysis is per serving.