Piano Roll, when recording, it (sometimes) plays the note twice

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### Bug Summary

When recording from MIDI devices on Piano Roll, sometimes I can **hear the same note twice**. It appears to be a random event, but I can reproduce it using a simple MIDI script.

#### Steps to reproduce

1. Create an empty project, and **add an instrument** (for example, an *AudioFileProcessor* with a kick sound as a sample).
2. Set the instrument **MIDI input** to your testing device (or launch the following script and use it as tester).
3. Open a Clip on Piano Roll, and **start recording**.

Script to test behavior:
```python
#!/usr/bin/env python3

import mido
import time

note = 69
port = mido.open_output(name="TEST", virtual=True)
on = mido.Message("note_on", note=note, velocity=127)
off = mido.Message("note_off", note=note, velocity=0)

while True:
print(".", end="", flush=True)

port.send(on)
time.sleep(5 / 1000)
port.send(off)

time.sleep(1)
```

#### Expected behavior

When a note is sent to the instrument, you **always** must hear it only once (not repeated). No arpeggiator, sequencer or any other effect used.

#### Actual behavior

Sometimes, you can hear the same note twice (with a very short delay between both *sounds*). In the Piano Roll, there is only one note recorded.

#### Screencast

https://user-images.githubusercontent.com/4654094/149574646-91a36c8b-0e0f-425a-b8b5-13ec5dad4e7c.mp4

#### Affected LMMS versions

* Version 1.3.0-alpha.1.206+g5a4dbf4cc (Linux/x86_64, Qt 5.12.8, GCC 9.3.0) on Ubuntu 20.04 / 21.04

Start with the Piano Roll recording flow and MIDI input behavior described in the issue, then reproduce it using the provided Python/mido script and an AudioFileProcessor instrument. Compare the audible playback with the single note shown in the Piano Roll. Done means each sent note is heard exactly once without an arpeggiator, sequencer, or other effect.

Written by the indexing model from the issue text.