Reduplication is often found in Latin irregular verbs of the third conjugation, usually in the perfect stem. This was the first way of forming the perfect tense in Latin, although it now only remains for a fraction of verbs.
Usually, the reduplicating syllable is made up of the first consonant of the verb's root with an 'e', e.g. pendo, pependi. However, the 'e' is often assimilated to the vowel of the verb root: momordi from mordeo, pupugi from pungo.
Compound verbs often drop the 'e' altogether: reppuli from repello. Also, many verbs weaken the vowel in the Perfect: cecini from cano, fefelli from fallo.
When the verb begins with 's' followed by another consonant, the reduplicating syllable begins with the two consonants, dropping the 's' in the middle (as my teacher would say 'to avoid the hiss'): spopondi from spondeo, steti from sto.