When the link between a word or sign and its meaning is already widely known, a poet's claim to exclusive priority is weak. The lion for courage and the sea for liberality are conventional examples.
An unusual image or an unfamiliar method of indicating a trait is different. Repetition of that detailed form by a later poet may warrant closer investigation.
This does not make every rare similarity proof of theft; it establishes that common agreement and distinctive agreement carry different evidential weight.