fix: remove duplicates in list-versions output

When we tell the provider to look at both sdists and wheels, and there
are both types of files, it returns some versions multiple times. Remove
any duplicates before printing the output.

Author: dhellmann

src/fromager/commands/list_versions.py

@@ -129,7 +129,7 @@ def list_versions(
         else:
             raise click.ClickException(f"No versions found for {req.name}")
 
-    versions: list[Version] = sorted(candidate.version for candidate in candidates)
+    versions: list[Version] = sorted(set(candidate.version for candidate in candidates))
     logger.info(f"Found {len(versions)} version(s)")
 
     for version in versions: