usually you wont be given an overall mark but its quite easy to work out. For example if you do 6 modules. 4 singles and 1 double, you count the double twice, for example if you scored 60,61,62,63 in teh four singles and 65 in the double. To work out the overall mark you do
(60 + 61 + 62 + 63 + (2 x 65) ) / 6.
Do basically a double module counts twice as much as a single and triple counts for 3 times as much.