Combine the honey, tamarind concentrate, lime juice, garlic and chiles in a saucepan, bring to simmer and cook for 15 minutes. Remove from the heat and let cool for 10 minutes. Transfer to a food processor, season with salt and pepper and process until smooth. Pour into a bowl and let cool.
Preheat a charcoal grill for medium-low and medium direct heat, and indirect grilling. Have ready a squeeze bottle filled with water to handle flare-ups.
Score the duck breasts with a sharp knife to make a diagonal cross-hatch pattern and sprinkle with salt and pepper. Place the duck on the grill, skin-side down, and cook, uncovered, until the skin is nicely browned, about 15 minutes. If the flare-ups are too great, or the duck browns too quickly, move the duck occasionally over the indirect heat, and then back to direct heat.
Once almost all of the fat has rendered and the skin is a deep golden brown, flip the duck over, move over medium heat, brush generously with the tamarind glaze and continue grilling until an instant-read thermometer reaches 140 degrees F for medium-rare, about 5 minutes. Remove the duck to a cutting board, brush with more of the glaze and let rest for 5 minutes before slicing on the diagonal into 1/4-inch-thick slices.
Spread some glaze on the corn tortillas. Top with some duck slices, Pineapple Relish, Pickled Onions and cilantro leaves.
Preheat a charcoal grill for direct medium-high heat.
Brush the pineapple slices with oil and sprinkle with salt and pepper. Grill the pineapple until charred on both sides and just cooked through, about 2 minutes per side. Remove the pineapple, cut into dice and place in a bowl. Add the green onions, serrano chiles, lime juice and 2 tablespoons canola oil and toss. Add the cilantro and toss again. Taste, season with salt and pepper, and add more oil, if desired. Let sit at room temperature for 30 minutes before serving.
Pickled Red Onions:
Combine the lime juice, vinegar, sugar and salt in a small saucepan, bring to a boil and cook until the sugar and salt dissolves. Remove from the heat and let cool for 5 minutes.
Pour the liquid mixture over the onions and mix to combine. Top the onions with a small plate to weigh down slightly, cover with foil or plastic wrap and refrigerate for at least 4 hours and up to 24 hours.