Can two different species have the same common name on inat?

Yes, and there’s no way of avoiding it.

For instance, different spider species live in different parts of the world and consequently the species typically encountered in houses vary geographically, meaning that the common name “house spider” refers to several different species globally. Likewise a “garden spider” refers to entirely different species depending on where you are. This issue applies to many thousands of species. It’s just one of the many many downsides of common names 🤷

Sure, you could argue that we should rename them all things like “Scottish house spider” and “New Zealand house spider” and “Western USA house spider” etc etc… but the point of common names is they are the names that are in common usage; such invented names are not in usage by anybody, so aren’t common names. If you’re going to invent new unique names for every species, then you might as well use scientific names, because that’s exactly what that system already does.