Bring plenty of water to a boil in a pot, then add duck legs and parboil for 2 minutes. Remove the duck and discard the liquid.
In a clean pot, add 6 cups of water, garlic cloves, ginger slices, scallion whites and salt. When the water starts to simmer, add duck legs. Let the water simmer again and add all the rice.
Bring everything to a boil, then lower heat to a gentle simmer. Skim off foam, then cover the pot with the lid askew. Simmer for about 30-35 minutes.
Check the congee pot every 10 minutes or so, give it a stir or skim off foam if needed. After 30-35 minutes, take the duck legs out of the pot. Seperate the meat from the bones. Shred the meat and put the bones back into the pot and simmer for another 30 minutes.
After 30 minutes, remove the bones from the congee. Taste and adjust seasoning with more salt or fish sauce. Also check the consistency and adjust it to your liking by adding a bit more water to thin it out or simmer it a little longer to thicken it.
Add shredded duck meat to the congee and stir to distribute evenly in the congee. Transfer congee to serving bowls and top with thinly sliced scallions, cilantro, Vietnamese coriander, julienned ginger and black pepper.