balletomane.

From ballet + -mane. Although the spelling balletomane is the most common nowadays, one would expect the spelling ballettomane, by analogy with other derivatives of nouns ending in the diminutive suffix -et, which commonly insert an additional t before vowel-initial suffixes (for example, sonnettiste, motettiste, juillettiste, cornettiste, flageolettiste, galoubettiste). Indeed, ballet derives from Italian balletto, a diminutive of ballo, formed with the Italian suffix -etto, which is a cognate of the French suffix -et (both deriving from the Late Latin suffixe -ittus or -ettus).