This discussion separates shared subject matter, distinctive expression, and authorship attribution. The evidential force of each differs.
Without a securely established text, one cannot confidently compare wording and arrangement. Likewise, recurrence of familiar imagery is not necessarily naskh or intihāl.
Advanced rhetorical criticism attends to textual accuracy, attributions, chronology and the exact form of correspondence. A possible resemblance should not be presented as a conclusive verdict.