Maybe a little complicared but you can still construct a case to trigger the double free action, thus causing a SIGABRT.

Program received signal SIGABRT, Aborted.
0x00007ffff7c3718b in raise () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007ffff7c3718b in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7c16859 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x000000000051e94e in fatal_error (prefix=prefix@entry=0x0, msg=msg@entry=0x6a38e4 "_PyObject_AssertFailed", status=status@entry=-1) at ../Python/pylifecycle.c:2183
#3  0x00000000005213b0 in Py_FatalError (msg=msg@entry=0x6a38e4 "_PyObject_AssertFailed") at ../Python/pylifecycle.c:2193
#4  0x0000000000470dd9 in _PyObject_AssertFailed (obj=obj@entry=0x7ffff789c350, expr=expr@entry=0x0, msg=msg@entry=0x6a392d "object has negative ref count", file=file@entry=0x691111 "../Include/object.h", line=line@entry=541, 
    function=function@entry=0x6a4a20 <__func__.15840> "_Py_NegativeRefcount") at ../Objects/object.c:2200
#5  0x0000000000470ee1 in _Py_NegativeRefcount (filename=filename@entry=0x691111 "../Include/object.h", lineno=lineno@entry=541, op=op@entry=0x7ffff789c350) at ../Objects/object.c:235
#6  0x0000000000490ad8 in _Py_DECREF (op=0x7ffff789c350, lineno=541, filename=0x691111 "../Include/object.h") at ../Include/object.h:473
#7  _Py_XDECREF (op=0x7ffff789c350) at ../Include/object.h:541
#8  type_mro_modified (type=type@entry=0xb17400, bases=bases@entry=0x7ffff5917890) at ../Objects/typeobject.c:343
#9  0x00000000004940ca in mro_internal (type=type@entry=0xb17400, p_old_mro=p_old_mro@entry=0x0) at ../Objects/typeobject.c:1961
#10 0x000000000048c816 in PyType_Ready (type=type@entry=0xb17400) at ../Objects/typeobject.c:5345
#11 0x0000000000493974 in type_new (metatype=<optimized out>, args=<optimized out>, kwds=0x0) at ../Objects/typeobject.c:2806