They are both in the same animal order but different families.
Dugons range from the Rea Sea & east coast of Africa to northern coast of Australia north to the Ryukyu Islands of Japan, while the Manatee ranges from Georgia coast to Gulf Of Mexico & the Caribbean to Brazil and up the
Amazon,
then again in West Africa from Mauritania south to Angola and inland as far as
Lake Chad.
I once lived in Florida & studied this in marine biology.
Mike