Robustness: make sure GBIF taxon key (for species) is unique

[niconoe]

No description provided.

[damianooldoni]

Why do you need this? Is a check on the downloaded data or on the taxon keys used in the API query?

[niconoe]

No, just for consistency. I was manually adding a list of species to the Pterois demo site, and sometimes I got unsure if I already added one 5 minutes prior or no. So I thought it might be good to add a constraint on that (unless there's a good reason to add several times the same GBIF key to a Pterois instance)