Yes - what you're seeing is part of the new dual build system, which lets you set up a second build independently of the first.
The second build will have the same primary and secondary power sets as the original, but you can choose a powers and slots in a different order and use different pools. The second build has no enhancements to start with, and you'll need to obtain and slot enhs for it separately.
And, to emphasise, no decision you make on the second build changes the first build in the slightest.
On the second build do you count as being lvl 1 again initially and thus can play though any arcs you missed the first time around getting XP etc as if it were a new toon again?
Nope, it's basically what would have happened if you leveled 1 to 50 without going to a trainer. Your Combat Level is 50, so you don't earn EXP and count as being over the level limit for arcs, IIRC
Do you need to lvle up the second build or just pick powers and slots up to your current level?
Level up. Unfortunatly it can be a time consuming process first time round. On a new toon, it's not too bad though.
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Level up. Unfortunatly it can be a time consuming process first time round. On a new toon, it's not too bad though.
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Not "level up" in the "do missions to earn xp" sense.
As others have said, the other build's at the exact same Combat Level (i.e. 50 in the OP's case) as the original build, it's just treated as if he'd never been to a trainer.
So the OP will have 50 levels-worth of power and slot picking for the second build as soon as he starts doing anything with it.
Sorry, was ambigious. 
You still need to go through a trainer manually assigning powers and slots, but don't need to gain the xp you can just burn through all 50 levels at once.
Cheers thats clear now.
Shame you can't level them up again, I'd effectively have 10 more slots 
Hello
What is Active Build about
I have a level 50 Controller
I was looking at Active Build 2 level 1
If I select Build 2 level 1
Will I still have my level set at 50 ?