If I think of duck then Hoisin sauce springs to mind although I have never seen it sold in rural Andalucía, however, with a bit of effort you can make your own. By the way I just happened to have an aging tin of black beans in a dark corner of my store cupboard.
This makes about 120ml which will keep in an air tight sterilised jar or bottle for a very long time.
6 tablespoons brown sugar
3 tablespoon black bean and garlic paste (3 tbs beans & 3 garlic cloves blitzed)
2 tablespoon runny honey
½ teaspoon Chinese five-spice powder
½ teaspoon tahini paste
¼ teaspoon sriracha
35ml soy sauce
70ml rice vinegar
Place all ingredients into a saucepan, bring to a boil then simmer for 5 minutes until the sauce thickens. Stir frequently.