Poetic borrowing becomes a question when an expression seems to transmit an earlier poet's distinctive idea or mode of representation. Two poets praising courage or generosity have not thereby demonstrated taking from one another.
The analysis separates three matters:
1. Purpose: which quality or aim is expressed?
2. Mode of indication: what image, allusion or arrangement conveys it?
3. Distinctiveness: is that mode common or an unusual achievement?
The distinction prevents a shared subject from being confused with a distinctive borrowed expression.