The word 哥哥 is formed through the process of reduplication, where the single character 哥 (which originally and independently means "older brother") is simply repeated. This doubling creates a term that is more affectionate, familiar, and commonly used in direct address than the single syllable. It is a straightforward, phonetically identical repetition for the purpose of forming a natural, disyllabic word that fits the rhythmic patterns of everyday spoken Chinese.