How to drag and drop an icon/skill from 1 slot to the other so that it don't duplicate on the next slot ?

I can drag and drop my skills on an actionbar, the problem tough is that if i move the skill from slot 1 to slot 2 for example it will just duplicate this skill from slot 1 to 2 so i have the same skill on both slots then. What i want is to be able to drag it from 1 (slot 1 should then be empty and then drop it on 2 where it should stay…

Here is how it looks like

Any help would be really nice and thx in advance