Matthew, Luke and John seem to be in agreement. The difference you're pointing out is in Mark where it looks like the same girl accused Peter two times in a row.
QUOTE:
Mark 14:69-70
The servant girl saw him standing there and began telling the others, "That man is definitely one of them!" Peter denied it again.
In Mark's account, that same girl is telling others that Peter was with Jesus, not accusing Peter to his face for a second time. When Peter denies it, he denies it to someone in that group and thereby to the whole group that was there listening, not necessarily that same girl, although that first girl was apparently part of that group. Matthew indicates it was to another girl in the group, Mark indicates to the group in general, Luke just says someone else, John neglects to mention it at all. All are true.
Maybe this link will help explain it better:
http://www.biblestudy101.org/apol/hbrecon1.html
It puts all the four Bible versions of this story side-by-side in the right column and explains what's going on in the left column. Hopefully that gives a better picture of what's going on. What do you think?