Cock Crowing Contradiction

Many passages in the Gospels represent the same events in somewhat different language. In these cases, it is important to remember that none of the accounts is exhaustive. Each author related only that information that was important to his particular point or perspective. Moreover, none of the biblical authors held to modern notions of accuracy -- none attempted to quote Jesus with absolute precision. Rather, they held to ancient standards of reliable testimony. They were interested in accurately reflecting meaning, not necessarily in recording speeches word-for-word.

In the case you present, the most likely explanation is that Mark quoted Jesus more directly, and that Matthew, Luke and John were interested only to preserve the fact that Jesus indicated that Peter would deny him before the cock crowed. Mark, writing from Peter's firsthand account, was probably more bent on preserving what Peter thought was important. Peter had a greater personal interest in this story than did the other writers, and he evidently remembered "twice" as significant. Perhaps "twice" added poetic emphasis to "three times" by showing the escalation in number (the Proverbs do this on occasion [compare Pss. 6:16; 30:15]).