Consider the following code:

import sys

from PySide6.QtCore import QObject
from PySide6.QtWidgets import QApplication, QWidget

class AnotherObject(QObject):
    pass

class MyWidget(QWidget, AnotherObject):
    def __init__(self, parent: QWidget | None = None) -> None:
        super().__init__(parent)  # RuntimeError: You can't initialize an PySide6.QtWidgets.QWidget object in class Window twice!

app: QApplication = QApplication(sys.argv)
MyWidget()

Where commented, the mentioned error occurs. Instead of QWidget, the MyWidget class may inherit any subclass of QObject but QObject itself, i.e., QComboBox, QLabel, QFrame, etc. Moreover, the issue reproduces with any two subclasses of the same Qt class, provided the first one is a Qt class, and the another is Pythonic, e.g., QLabel and any derivative of QWidget.

The error does not show up on PyQt5, PySide2, or PyQt6.