Two ancient texts use a strikingly similar phrase. The resemblance is worth investigating, but “one copied the other” is a larger claim. A useful first note records the similarity without deciding the entire relationship.
Define what actually matches
Are you comparing a single word, an image, a sequence of ideas or an extended passage? Record the locations in the editions you used. Keep enough context to show what each passage is doing.
If you are reading translations, note that the apparent match is in those translations. You have not yet established that the underlying wording matches in the same way.
Test more than one explanation
For an invented example, two stories both describe a lamp in a dark room. That observation alone does not establish direct dependence. You would need a fuller argument about wording, context, chronology and other possible relationships.
Do not turn this caution into a claim that dependence can never be demonstrated. It means the conclusion should follow evidence beyond a resemblance selected because it looks persuasive.
Keep a comparison note
Use four fields: passage A; passage B; precise similarity; questions still unresolved. Then consult scholarship that addresses the relationship directly, checking what evidence its authors use.
Our guide to comparing translations is useful when the words first caught your attention in English. Careful comparison starts with a small, reproducible observation and leaves room for the argument that comes next.