Not every shared theme proves plagiarism. Courage, generosity, beauty and separation are common poetic subjects that multiple writers may address independently. The analysis becomes more specific when distinctive wording, unusual imagery, chronological priority and demonstrable borrowing are present.
Naskh and intiḥāl concern taking another’s utterance and claiming it; ighārah and maskh involve particular verbal modifications; and ilmām primarily concerns taking a meaning.
On encountering similar verses, separately establish evidence of borrowing, similarity of wording and distinctiveness of the shared idea. Resemblance alone does not determine the judgment.