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Environment: Win 10, Arduino 1.8.13, Genuine Arduino Mega2560
The upload function seems to start the mega2560 briefly then restarts it.
In the example sketch it results in a double output of "Hello" instead of a single output.
This problem does not exist in 1.8.12 and earlier versions.

// example mega2560 problem
void setup()
{
 Serial.begin(115200);
 while (!Serial);
 Serial.println("Hello");
}
void loop() {}