Cancelled Builds and Energy Store (Build queue?)

This may just be me not understanding something but If you issue another build order while a structure is being built, the first is left partially built. I cannot find a way to complete the first half built structure. I would say either it should wait till the first was finished (some sort of build queue), alert that you cannot do it yet or something along those lines.

This also causes a problem when I try to build an energy store as even when it is left to build it stops about 2% short and the building is not completed

Issuing an order will always override the existing task, unless you're holding down shift which will queue the task after the current one and previous orders complete.

Ok, thats fine, but should i be able to complete a half finished building at a later stage?

You just need to right click on it with a fabrication unit (commander or otherwise). You can guarantee it'll continue building by clicking on the "repair" command on the right side bar (blue cross) and left clicking on the building you want to continue building.

Right click is always a context sensitive command based on what is below the cursor and the unit you have selected.