Hey,
I found the following on this website
http://registrar.mcmaster.ca /calen...-13/pg121.html
Repetition of Courses: Students may repeat courses that have been failed or for which credit has been obtained a number of times, with the exception of the students in the Faculty of Business who may only repeat courses which they have failed. The grades for all attempts appear on the transcript and enter into the computation of the Cumulative Average. However, only one successful attempt will enter into the computation of credit earned towards your degree.
Based on this I would think both marks will be equally weighted in your GPA calculation, otherwise they would mention something about it.
It would be best if this was confirmed by someone who had repeated a course before.