Rare agreement can strengthen a suspicion of borrowing, but possibility is different from proof. Chronology, the later poet's possible access and the exact location of the unusual similarity can all matter.
The core critical principle is that a common topic must not be equated with a distinctive invention. “Both praised generosity” is weak evidence; “both used an identical unusual arrangement” is a more substantial question.
This avoids both unexamined exoneration and automatic accusations based on resemblance.