How to … Tell if a generic name already exists
When publishing the name of a new genus, it is important to ensure that the name you want to use has not already been validly published for a different genus; if it has, your new name will be an illegitimate later homonym. Note that this will be the case even if the earlier homonym has not been used for many years, or has never been accepted, or is an illegitimate or rejected name; if a name has already been validly published, any later homonym will be illegitimate.
A convenient way to check for published generic names for all organisms covered by the Code (including fossils) is to use Index Nominum Genericorum (ING), an online database of published generic names maintained by the Smithsonian's National Museum of Natural History, Washington.
Note that homonymy (Art. 53) applies to names across all groups of organisms covered by the Code. For example, a fungus and a flowering plant, or a moss and a red alga, cannot have the same name.
Homonymy does not apply across Codes (e.g. a plant and an animal can have the same name; Art. 54.1), except that a new fungal name published on or after 1 January 2019 will be illegitimate if it is a later homonym of a prokaryotic or protozoan name (Art. F.6.1). In any event, it is best practice to avoid cross-Code homonymy. Unfortunately, there is no equivalent to Index Nominum Genericorum for animals. However, the Encyclopedia of Life, Catalogue of Life and Global Biodiversity Information Facility can be useful for checking animal names.
For more information, see Chapter 5 How to publish a new name in The Code Decoded,