This is the special ed teacher in me talking (Anyone who has experience with Wilson reading or a similar program will understand this response
There are six types of syllables in the English language, including open and closed syllables. When dividing up Rebeca into syllables it would look like this: Re be ca. The first e is a schwa (uh sound) which happens on unstressed open syllables. The second e would be pronounced the long way (ee) because it is an open syllable and bears the stress, and the last syllable is also a schwa since it is unstressed.
The second c serves as a buffer letter, closing in that syllable so that the e says "eh". So it would look like this: Re bec ca. (When dividing syllables you divide between two consonants unless they are digraphs).
So in the case of a name like Rebecca, the second c does serve a purpose, it's not just there for decoration.