«خاط لي عمرو قباءً، ليت عينيه سواء»
ʿAmr sewed a cloak for me; would that his two eyes were alike.
If one of his eyes is healthy and the other impaired, “alike” could mean that both become healthy or that both become impaired. The first reading is a blessing, the second a curse.
The words are identical. What changes is the eye taken as the standard of comparison. Two possible comparisons produce opposed outcomes within one utterance.