For example the RRA two stage trigger. Why does it have the take-up (stage one). Is the second stage too light for safety purposes? Does anyone manufacture a replacement trigger that breaks like a standard AR trigger and has the pull weight of a two stage? If so is it safe for duty use?

it's so you can put your finger on the trigger and walk around with the safety off and "feel safe".  it's not to light, it's someone's idea of being "safe".  the same basic idea as that the Glock uses.  i like my JP match single stage trigger.  do a search on JP enterprises.  it's very safe as long as you keep the safety on and your finger off the trigger until you are ready to fire.  when you are done firing put the safety back on and take your finger off the trigger.

Two stage triggers aren't safety devices so you can run around with your finger on the trigger.  

4.5# two stage triggers trip at 4.5# just like 4.5# single stage.  What they do is provide an accurate means of judging the point just before the sear breaks and the rifle fires, important in precision riflery.

A 4.5# single stage trigger takes 4.5# of pressure all at once to trip.

A typical 4.5# two stage trigger can have 3.5# of pressure placed on it causing the trigger to move to a point you can feel with your trigger finger;  an [i]additional[/i] 1# of pressure will trip the sear.  The additional 1# is all you feel a the end, a very light trigger pull and one that can score X-rings at Camp Perry with the right rifle, shooter, and ammo.

I'll be surprised if there are any competitors in Service Rifle at Camp Perry with single stage triggers, certainly no one in the Presidents Hundred.  

US Military rifles since the M189x Krag (and maybe before -- until the M16, of course) have had two stage triggers which means they're suitable "field" as well as range triggers.  They're neither better nor worse than single stage in the field.

I've hedged my bets [;)].  I have Accuracy Speaks (single stage) and ArmaLite (2-stage) triggers in two of my semi-only rifles.  My #1 carbine has the Accuracy Speaks.

-- Chuck