AsyncSequencerClient: handle only 1 input at time

Sometimes the I/O callback could be called more than once before the
future is read (why?) – ignore those calls.

Author: Jajcus

alsa_midi/client.py

@@ -1796,6 +1796,8 @@ async def event_input(self, prefer_bytes: bool = False, timeout: Optional[float]
         fd = self._fd
 
         def reader_cb():
+            if fut.done():
+                return
             result = None
             try:
                 result, event = self._event_input(prefer_bytes=prefer_bytes)
@@ -1839,6 +1841,8 @@ async def _event_output_wait(self, func, timeout: Optional[float] = None) -> Uni
         fd = self._fd
 
         def writer_cb():
+            if fut.done():
+                return
             nonlocal remainder
             result = None
             try: