The Section Divider is an element that located inside the section element. Line between Dividers could appear in case there is a space between the sections

At first, make sure that there is no custom space added in Design settings of the the sections in Spacing and Transforms option groups:
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Custom CSS code could also cause this issue. You will need to remove all the custom code from the section settings, page settings and Theme Options to check if the issue is related to any custom code or not.

Section Settings:

Page Settings:

Theme Options: