feaLib: Dedupe multiple substitutions with classes

Lib/fontTools/feaLib/ast.py

@@ -1272,7 +1272,10 @@ class MultipleSubstStatement(Statement):
             replaces.append(replace)
         replaces = list(zip(*replaces))
 
+        seen_originals = set()
         for i, original in enumerate(originals):
+            if original not in seen_originals:
+                seen_originals.add(original)
                 builder.add_multiple_subst(
                     self.location,
                     prefix,