This is, unfortunately, a known behavior. The solution is to ensure that you have an album in the library that has the proper artist credit already, before you import the new local files. Alternatively, you can later merge the duplicate artist into the proper one, my selecting both in the artist view or in the search results and clicking the Merge button that will appear.
See here for more details (and in the posts that are linked from there):
Sometimes it also happens that an artist already exists twice (or more) in the metadata that Roon receives. Then you can also merge them. (You need to have at least one album with each artist credit in the library). You can report it to Roon in the Metadata forum category.