Due dates for routines are dictated by annual date and not the start date itself. When a routine has levels based on weeks and are less than a month (weekly/fortnightly/tri-weekly) you may see duplications on the routine schedule within a given month like the example below:
The example here is showing the routine services due each month based on a service level frequency of 3 weeks. There are 2 routine services due in the same month for June and August, while April, May and July only have 1.
This is a normal occurrence, as a tri-weekly level is likely to have more than 1 routine service due if the the due date happens at the beginning of the month. Since calendar months vary in the total number of days, it's not unusual to see a pattern like the example above.
If you wish to check that the due dates are lining up correctly you can use a date calculator and input the weeks (in number of days) so you can see what the correct dates are and match them to the dates in the system.