My skill add on is not giving correct duration. For example for movement speed the duration is supposed to be 12 seconds. If you have that add on on two different skills the second one will have a duration of only 7 seconds. Now if you wanted to change the add on to a different skill and got rid of the movement 12 second skill you can no longer get 12 seconds for that skill. It will only be 7 seconds. The same goes for all the add on's with 12 second durations.

Skill addons durations are based on the CD of the skill they are added to. Here is an overview:

If you mean something else, can you attach some screenshots so we can understand what is going on?