Substrate

"Moonmoon is an example of the linguistic process of reduplication, which is often deployed in English to make things more cute and whimsical. In the pure form of reduplication, you get words like bonbon, choo-choo, bye-bye, there there, and moonmoon but relaxing the rules a little to incorporate rhymes and near-rhymes yields hip-hop, zig-zag, fancy-shmancy, super-duper, pitter-patter, and okey-dokey. And with contrastive reduplication, in which a word repeats as a modifier to itself"