The RHS is the authority for liliums. On Line Register is here:
http://www.lilyregister.com/
Register supplements are on the RHS site in PDF form.
Often the duplicate names represent lilies in different divisions. Sometimes a lily will go into the register but there will be a disclaimer that the name (epithet) is unacceptable because the name is already in use. Co-op lilies tend to generate a lot of misnamed lilies but the buyers don't know that. That is why we have moderators here, but the job gets tough sometimes. Then as you say, there are the Dutch.
.
