fix: avoid increase the set->used twice when the __eq__ trigger another add

Signed-off-by: Keming <kemingy94@gmail.com>

Author: kemingy

Misc/NEWS.d/next/Core_and_Builtins/2025-11-27-17-47-09.gh-issue-141805.TT0biF.rst

@@ -0,0 +1,2 @@
+Avoid increase the PySetOjbect->used twice when adding one element triggers
+__eq__ that cause another set.add

Objects/setobject.c

@@ -180,6 +180,7 @@ set_add_entry_takeref(PySetObject *so, PyObject *key, Py_hash_t hash)
             else if (entry->hash == -1) {
                 assert (entry->key == dummy);
                 freeslot = entry;
+                goto found_unused_or_dummy;
             }
             entry++;
         } while (probes--);