Poetic borrowing cannot be inferred from a shared general theme alone. Generosity, courage, separation, death and longing are widespread subjects; their occurrence in two poets does not by itself prove verbatim taking, adapted appropriation or conceptual borrowing.
The investigation must look for a distinctive imaginative connection, an unusual form of presentation or particular wording that may have been transferred; then assess how much the later poet transformed it and whether the result is rhetorically better or worse. Historical attribution requires its own evidence.
A classical literary label should not be converted into a conclusive historical accusation without support, nor automatically identified with a modern legal judgement about plagiarism.