building: add_suffix_to_extensions: avoid reprocessing entries

Detect if EXTENSION entry has already been processed and avoid
processing it again, which would mangle the filename due to
replacement of dots in suffix with os.sep (resulting in pyinstaller#5851).

PyInstaller/building/utils.py

@@ -98,6 +98,14 @@ def add_suffix_to_extensions(toc):
     new_toc = TOC()
     for inm, fnm, typ in toc:
         if typ == 'EXTENSION':
+            if fnm.endswith(inm):
+                # If inm completely fits into end of the fnm, it has
+                # already been processed
+                new_toc.append((inm, fnm, typ))
+                continue
+            # Check if the extension entry has already been processed.
+            # This should be reliably indicated by presence of os.sep
+            # in the inm... Unless
             # Change the dotted name into a relative path. This places C
             # extensions in the Python-standard location.
             inm = inm.replace('.', os.sep)