Make sure extensions are unique in MarkItDown's convert methods. (#1076)

Author: afourney

packages/markitdown/src/markitdown/_markitdown.py

@@ -381,7 +381,8 @@ def _append_ext(self, extensions, ext):
         ext = ext.strip()
         if ext == "":
             return
-        # if ext not in extensions:
+        if ext in extensions:
+            return
         extensions.append(ext)
 
     def _guess_ext_magic(self, path):