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The Arduino "language" is nothing more than a set of C++ functions and classes. It does not mandate any particular C++ standard.
The standard is dictated purely by the compiler that the core you are using happens to support (and is configured to use through command line flags).
Different cores use different compilers. Different versions of the same ...
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This is NOT my answer, this is the answer of @Gerben, thank you.
The Arduino IDE suspends the serial connection while uploading, and then resumes when done. It looks like they added some delay before reopening the serial connection. So it looks like your code will run twice, even when you keep the Serial Console open
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The problem is most likely your power supply coupled with the poor design of the capacitive sensing system.
The capacitive sensing is done by measuring the capacitance between your foil plate and "ground". That "ground" is usually a connection through the USB cable and your computer's power supply through the earth wire of your power ...
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