Lojban doesn't make any distinctions in length, so lengthening the "c" sound won't change anything. So, yes, phonologically it's valid. Orthographically, however, it isn't. The strict form of the writing system doesn't let two adjacent consonants be identical. Obviously, most people can understand less restricted writing forms, just as most people can understand "Whassssssup?", even though, strictly speaking, it isn't English.