إذا اتفق القائلان على الغرض العام
When two speakers agree on a broad theme.
Courage, generosity, beauty and modesty are widely understood qualities. Calling someone brave or generous is not the invention of a single poet. Agreement on such a theme is therefore insufficient to establish borrowing.
If, however, both poets use a markedly distinctive image or reasoning to convey it, the inquiry moves from the theme itself to the way it is expressed.