Make CConverter.add_include() smarter

Don't fail when adding exactly the same include twice.

Author: vstinner

Tools/clinic/clinic.py

@@ -3335,9 +3335,12 @@ def parser_name(self) -> str:
 
     def add_include(self, name: str, reason: str,
                     *, condition: str | None = None) -> None:
+        include = Include(name, reason, condition)
         if self.include is not None:
+            if self.include == include:
+                return
             raise ValueError("a converter only supports a single include")
-        self.include = Include(name, reason, condition)
+        self.include = include
 
 type_checks = {
     '&PyLong_Type': ('PyLong_Check', 'int'),