diff --git a/sdv/validators/xml_schema.py b/sdv/validators/xml_schema.py
index 690d267..206a402 100644
--- a/sdv/validators/xml_schema.py
+++ b/sdv/validators/xml_schema.py
@@ -293,7 +293,7 @@ def _walk_schemas(self, schema_dir):
             schema file paths.
 
         """
-        seen = []
+        seen = set()
         schemalocs = collections.defaultdict(list)
 
         for top, _, files in os.walk(schema_dir):
@@ -308,7 +308,7 @@ def _walk_schemas(self, schema_dir):
                     continue
 
                 schemalocs[target_ns].append(fp)
-                seen.append((target_ns, fn))
+                seen.add((target_ns, fn))
 
         for ns, loc in self.OVERRIDE_SCHEMALOC.items():
             schemalocs[ns] = [loc]