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

@@ -1,160 +1,161 b''
 """Infrastructure for registering and firing callbacks on application events.
 
 Unlike :mod:`IPython.core.hooks`, which lets end users set single functions to
 be called at specific times, or a collection of alternative methods to try,
 callbacks are designed to be used by extension authors. A number of callbacks
 can be registered for the same event without needing to be aware of one another.
 
 The functions defined in this module are no-ops indicating the names of available
 events and the arguments which will be passed to them.
 
 .. note::
 
    This API is experimental in IPython 2.0, and may be revised in future versions.
 """
 
 from backcall import callback_prototype
 
 
 class EventManager(object):
     """Manage a collection of events and a sequence of callbacks for each.
     
     This is attached to :class:`~IPython.core.interactiveshell.InteractiveShell`
     instances as an ``events`` attribute.
     
     .. note::
 
        This API is experimental in IPython 2.0, and may be revised in future versions.
     """
     def __init__(self, shell, available_events):
         """Initialise the :class:`CallbackManager`.
         
         Parameters
         ----------
         shell
           The :class:`~IPython.core.interactiveshell.InteractiveShell` instance
         available_callbacks
           An iterable of names for callback events.
         """
         self.shell = shell
         self.callbacks = {n:[] for n in available_events}
     
     def register(self, event, function):
         """Register a new event callback.
         
         Parameters
         ----------
         event : str
           The event for which to register this callback.
         function : callable
           A function to be called on the given event. It should take the same
           parameters as the appropriate callback prototype.
         
         Raises
         ------
         TypeError
           If ``function`` is not callable.
         KeyError
           If ``event`` is not one of the known events.
         """
         if not callable(function):
             raise TypeError('Need a callable, got %r' % function)
         callback_proto = available_events.get(event)
-        self.callbacks[event].append(callback_proto.adapt(function))
+        if function not in self.callbacks[event]:
+            self.callbacks[event].append(callback_proto.adapt(function))
     
     def unregister(self, event, function):
         """Remove a callback from the given event."""
         if function in self.callbacks[event]:
             return self.callbacks[event].remove(function)
 
         # Remove callback in case ``function`` was adapted by `backcall`.
         for callback in self.callbacks[event]:
             try:
                 if callback.__wrapped__ is function:
                     return self.callbacks[event].remove(callback)
             except AttributeError:
                 pass
 
         raise ValueError('Function {!r} is not registered as a {} callback'.format(function, event))
 
     def trigger(self, event, *args, **kwargs):
         """Call callbacks for ``event``.
         
         Any additional arguments are passed to all callbacks registered for this
         event. Exceptions raised by callbacks are caught, and a message printed.
         """
         for func in self.callbacks[event][:]:
             try:
                 func(*args, **kwargs)
             except (Exception, KeyboardInterrupt):
                 print("Error in callback {} (for {}):".format(func, event))
                 self.shell.showtraceback()
 
 # event_name -> prototype mapping
 available_events = {}
 
 def _define_event(callback_function):
     callback_proto = callback_prototype(callback_function)
     available_events[callback_function.__name__] = callback_proto
     return callback_proto
 
 # ------------------------------------------------------------------------------
 # Callback prototypes
 #
 # No-op functions which describe the names of available events and the
 # signatures of callbacks for those events.
 # ------------------------------------------------------------------------------
 
 @_define_event
 def pre_execute():
     """Fires before code is executed in response to user/frontend action.
     
     This includes comm and widget messages and silent execution, as well as user
     code cells.
     """
     pass
 
 @_define_event
 def pre_run_cell(info):
     """Fires before user-entered code runs.
 
     Parameters
     ----------
     info : :class:`~IPython.core.interactiveshell.ExecutionInfo`
       An object containing information used for the code execution.
     """
     pass
 
 @_define_event
 def post_execute():
     """Fires after code is executed in response to user/frontend action.
     
     This includes comm and widget messages and silent execution, as well as user
     code cells.
     """
     pass
 
 @_define_event
 def post_run_cell(result):
     """Fires after user-entered code runs.
 
     Parameters
     ----------
     result : :class:`~IPython.core.interactiveshell.ExecutionResult`
       The object which will be returned as the execution result.
     """
     pass
 
 @_define_event
 def shell_initialized(ip):
     """Fires after initialisation of :class:`~IPython.core.interactiveshell.InteractiveShell`.
     
     This is before extensions and startup scripts are loaded, so it can only be
     set by subclassing.
     
     Parameters
     ----------
     ip : :class:`~IPython.core.interactiveshell.InteractiveShell`
       The newly initialised shell.
     """
     pass