Two questions should not be collapsed:
1. Was a distinctive word, idea or mode of expression carried from the earlier verse?
2. What happened to eloquence when the material was recast?
The first establishes the nature of borrowing; the second assesses the artistry of the recasting. A later poet may improve an earlier distinctive idea; in another case, two poems may share only an ordinary theme and establish no taking at all.
Separating the questions prevents praise or blame from becoming an unsupported allegation.