ctypes: Return existing pointer when possible

Addresses pythongh-46376

Modules/_ctypes/_ctypes.c

@@ -5129,6 +5129,7 @@ static PyObject *
 Pointer_get_contents(CDataObject *self, void *closure)
 {
     StgDictObject *stgdict;
+    PyObject *keep, *pointer_probe, *pointer_to_pointer;
 
     if (*(void **)self->b_ptr == NULL) {
         PyErr_SetString(PyExc_ValueError,
@@ -5138,6 +5139,27 @@ Pointer_get_contents(CDataObject *self, void *closure)
 
     stgdict = PyObject_stgdict((PyObject *)self);
     assert(stgdict); /* Cannot be NULL for pointer instances */
+
+    keep = GetKeepedObjects(self);
+    if (keep != NULL) {
+        // check if it's a pointer to a pointer:
+        // pointers will have '0' key in the _objects
+
+        pointer_probe = PyDict_GetItemString(keep, "0");
+
+        if (pointer_probe != NULL) {
+            pointer_to_pointer = PyDict_GetItemString(keep, "1");
+            if (pointer_to_pointer ==  NULL) {
+                PyErr_SetString(PyExc_ValueError,
+                "Unexpected NULL pointer in _objets");
+            }
+            // don't construct a new object,
+            // return existing one instead to preserve refcount
+            Py_INCREF(pointer_to_pointer);
+            return pointer_to_pointer;
+        }
+    }
+
     return PyCData_FromBaseObj(stgdict->proto,
                              (PyObject *)self, 0,
                              *(void **)self->b_ptr);