No poet can claim ownership of an ordinary simile or ethical commonplace simply because he used it. Priority is meaningful only where a sufficiently distinct construction or thought is involved.
Precedence concerns identifiable novelty. Addition or improvement concerns what the later writer actually contributed: greater precision, stronger effect, a more apt image, or a better arrangement.
Being first and being more effective are different judgments. A later poet may lack priority yet create a genuinely stronger rhetorical treatment.