Since you say the call sounded like a Mallard duck, the first thing that comes to my mind is the Wood frog (Rana sylvatica, more recenty Lithobates sylvaticus). Most people who hear the call of this frog, often mistake it for the sound of a duck (I've done that lol ;P). Here is a link to the species profile.....if you scroll down you can click to hear a chorus. Wood Frog - Rana sylvatica I tried to find a single Wood frog recording but had no luck. Did the call sound similar to that?
I know there are Wood frogs in Georgia (more north than Atlanta though) but maybe the Wood frog population there has increased? The link above was last updated in 2008.
Wood frogs are very hardy creatures and will mate when snow is still on the ground or even when ice is still in the pond! I think the breeding season for the Wood frogs in Georgia in Jan-Feb; but maybe some of the frogs are coming out of hibernation early.....I've observed a similar behaviour in the WFs here in Alaska.
I always love trying to solve frog related mysteries hehe!!