This has been requested before: http://help.doit.im/topics/202
But it seems it is not implemented yet.

IMO, this is crucially important for any todo/task management app.
Basically most of the available apps ignore the fact that missing a reminder is very very likely to happen and they don't offer a decent solution for it.

So basically I would like reminders to repeat until I complete (check) the task. The repeating reminder should be optional and the interval of repeat selectable by me.
This means that there probably should be a setting that makes reminders repeat every few minutes/hours if their related task is not marked as complete/done.
As an example, this app (https://play.google.com/store/apps/details?id=com.dg.gtd.android.lite) is handling this feature in a great manner.

This feature plus the option to color code the projects/tags/contexts are the major missing features of this otherwise wonderful app.