When Amy calls somebody or something, she uses two syllables and a descending minor third(? — roughly from G to E on a C-major scale, I think).
If what she's calling only has a one-syllable name, she'll repeat the syllable in order to call it. This means that some nouns are homophones in the vocative but not the nominative case, I suppose :)
This is most noticeable with "Ma-ma" and "Fi-fi", since the first can be either "Mama" (Mummy) or "Ma" (Martin; he of the default userpic), and the second can be either "Fi" (a Miffy doll that Stella sewed for her) or "Fifi" (Sophie, a… hand-puppet might be the name for it; a big doll where you can put your arms into hers to move them or your hand into the mouth).