Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 3.0
Situational Awareness; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert
ATC Equipment / Nav Facility / Buildings; Human Factors; Airspace Structure
I took a handoff from a sector on Aircraft X.  I was coordinating with one of the towers in the airspace when who I believed was Aircraft X checking on.  After I got off the land line I answered Aircraft X with the local Altimeter and a descent to 8000 feet.  The female pilot responded with the descent to 8000.  I missed the call sign on the read back.  A couple of minutes go by and Aircraft Y checks on and I ask didn't we just speak.  The male pilot says no we are just now checking on.  I said ok and gave them the altimeter and the descent to 8000 feet.  Then I asked on the frequency something to the effect of is there a female pilot on descending to 8000 feet.  I didn't hear any reply.  I asked the Aircraft X if they heard a reply to that.  They said yes and he either told me or I asked what their call sign was.  I asked him to relay and get their position.  He replied with they are 20 miles south of Ashville.  I was stunned that they were so far away.  I had Aircraft X relay to have them maintain their previously cleared altitude and return to their last assigned frequency.  They didn't know their last assigned frequency so we found it for them and relayed it.  First and foremost I should have caught the call sign on the read back.  There were so many things that happened in order for this event to take place but if I had caught the read back it could have been saved.  First the timing of when Aircraft X was shipped and ended up on my frequency had to be at the exact time that it seemed appropriate to me the Aircraft Y would be checking in.  The pilot of the Aircraft X either was issued or dialed in the wrong frequency.  Then when I issued the descent and the altimeter she did not catch that I said Aircraft X or my sector local altimeter.  Also they had to be close enough to their destination that a descent from the flight levels seemed appropriate at that time for them to not question it.  The frequency had to work all the way to North Carolina; this is surprising because it doesn't work well in the entirety of our own airspace.  Again all of these things had to happen for this event to take place.  I should have caught the read back of the call sign and it would have been avoided.  The only technique I could have used to prevent this from happening would have been to not descend Aircraft X on the initial call.  Those aircraft coming over from the center are descending with a crossing restriction and I could have waited and not given them a further descent at that time.
A Controller erroneously assumed an aircraft checking in was an expected handoff from the previous sector and issued a descent clearance. The error was made evident after a second aircraft checked in with what the Controller thought of as the same call sign. A corrective clearance was issued to return the original aircraft to its intended route.