If you use real names for the permalinks and create multiple pages with the same page name, a number is automatically inserted after the URL. If this number appears even if there are no other pages with this name, this may be due to an image.
If an image has this particular name, an attachment page is automatically created, which then leads to this problem.
Solution: Simply change the image name, e.g. with Enable Media Replace and then adjust the URL in question.