A player occupying a Resource hex with a habitable site (section 8.3) may make a domestication attempt, even if the hex already has a metropolis, and even if the resource has previously been exploited.
I should clarify that, that only applies to resources. If animal domestication occurs,
The player gains the Infrastructure Advances listed, and he converts one of his units in the hex into a Metropolitan unit.
If a Plant or Animal hex is free of metropolises and its site is habitable ..
you cannot repeatedly attempt it in the same hex, as it produces a metropolis once successful. This also applies to crops, as they also produce a metropolis.