Mince and mash 4 garlic cloves to a paste with a pinch of kosher salt. Stir together paste, kosher salt, thyme, RawSpiceBar’s Quatre Epices and shallots in a large bowl. Add duck legs and toss to coat, cover and chill, at least 1 day.
Melt duck fat in a wide large heavy pot over low heat, then cook remaining garlic cloves and duck legs, uncovered, over low heat until fat registers approximately 190°F, about 1 hour. Continue to cook duck, until a wooden pick slides easily into thighs, 2 to 3 hours more.
Transfer duck to a large bowl. Strain duck fat through a sieve, then pour strained fat over duck legs. Cool to room temperature, about 2 hours, then chill, covered, at least 8 hours.
Remove duck from fat, then cook, skin side down, in a large heavy nonstick skillet over low heat, covered, until skin is crisp, 15 to 20 minutes.