Do I announce the target creature when I cast Duplicant? And if so, can my opponent sac the creature I target before Duplicant comes into play thereby making Duplicant simply a 2/4 creature?
You declare the target when Duplicant's trigger goes onto the stack; not when you cast Duplicant. But, your opponent can still respond to that trigger, knowing the target, to remove it first. In that case, the trigger won't resolve, nothing will be exiled, and your Duplicant will just be a 2/4 Shapeshifter.
Duplicant: Can opponent remove target?