When I enter the total project duration column ( customer field ) in to my project plan, it calculated total time duration for the tasks without taking activity sequencing into consideration. For an example, Activity 1 is taking 9 Days and Activity 2 and Activity 3 will take 4 days each, but here Activity 2 and Activity 3 has start to start sequence. When it comes to total duration is shows 17 days. But it ideally should be 13 days. Please provide your support on this.
Based on the description of the plan, the total duration comes to 13 days, and if excluded 4 days of weekend that it comes to 9 working days. That seems to be pretty small and straight sequence of activities.
Probably you may have to reset your calendar to a standard one and try again.