Repeating tasks, do they add on?
 

I want to have for example this kind of scenario:

A repeating task, for paying a bill, on the first of every month. However if I don't complete the task for the whole month, and now it's the first of the next month, I want to have 2 of the same task now, the original one that is now overdue, and the new one which is due now.

Is this possible? At the moment I can't figure out how to get a task to repeat without completing it.

Yeah, the way OmniFocus currently handles repeating actions is driven by completion of an action. However, if you are determined to have them "stack up" like this, you can get around it by having a number of parallel repeating actions. For your monthly bill, you could have a pair of repeating actions, each repeating every 2 months, one for the even numbered months and one for the odd. Put start dates on so they don't show up prior to being needed. If you wanted to be able to have more of them stack up, do a trio repeating every 3 months, or a quartet every 4 months, I trust the pattern is becoming clear :-)

Pay electric bill Start: 1 Mar 11 Due: 3 Mar 11 Repeat: every 2 months
Pay electric bill Start: 1 Apr 11 Due: 3 Apr 11 Repeat: every 2 months