Fixed broken code in last merged main, as we need to avoid duplicate entries and potential implicit joins due to identical table names

Author: kiraksi

sqlalchemy_bigquery/base.py

@@ -275,7 +275,8 @@ def _known_tables(self):
         asfrom_froms = self.stack[-1].get("asfrom_froms", [])
         for from_ in asfrom_froms:
             if isinstance(from_, Table):
-                known_tables.add(from_.name)
+                if from_.name not in known_tables:
+                    known_tables.add(from_.name)
 
         return known_tables