Kacc. example, "parakkamo"

Vidyabhusana explains _parakkamo_ as pa+a+kamo (with the "r" as a
euphonic insertion); the more obvious explanation would be para+kamo.
None of my dictionaries treat the word (perhaps because the prefix
"para" is self-evident) --is there a good reason for Vidyabhusana  to
propose pa+a rather than para?