Don't register the same callabck multiple times

Using the %matplotlib magics would not always work without this fix: 
For `%matplotlib qt5` to work, `flush_figures` should not be in `get_ipython().events.callbacks['post_execute']`. Yet without this fix:
```python
Python 3.7.6 (default, Jan  8 2020, 20:23:39) [MSC v.1916 64 bit (AMD64)]
Type "copyright", "credits" or "license" for more information.

IPython 7.18.1 -- An enhanced Interactive Python.

In [1]: %matplotlib inline

In [2]: %matplotlib inline

In [3]: %matplotlib inline

In [4]: %matplotlib inline

In [5]: get_ipython().events.callbacks['post_execute']
Out[5]: 
[<bound method AutoreloadMagics.post_execute_hook of <autoreload.AutoreloadMagics object at 0x00000199BFE73888>>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function matplotlib.pyplot.install_repl_displayhook.<locals>.post_execute()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>]

In [6]: %matplotlib qt5

In [7]: get_ipython().events.callbacks['post_execute']
Out[7]: 
[<bound method AutoreloadMagics.post_execute_hook of <autoreload.AutoreloadMagics object at 0x00000199BFE73888>>,
 <function matplotlib.pyplot.install_repl_displayhook.<locals>.post_execute()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>,
 <function ipykernel.pylab.backend_inline.flush_figures()>]

In [8]: 
```

IPython/core/events.py

@@ -60,6 +60,7 b' class EventManager(object):'
         if not callable(function):
             raise TypeError('Need a callable, got %r' % function)
         callback_proto = available_events.get(event)
+        if function not in self.callbacks[event]:
         self.callbacks[event].append(callback_proto.adapt(function))
     
     def unregister(self, event, function):