Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 6
Workload; Confusion; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Company Policy; Human Factors; Procedure
I was being relieved off of Departure Radar North (DRN) when Aircraft X departed flashing 'DM' (Departure Message) in the data block. I hurried to finish the relief briefing so I could troubleshoot. When I did; I found Aircraft Y inbound to ATL.  [Airline Company] had two aircraft with the same call sign in the air at the same time.  We advised our Aircraft X he was now going to be Aircraft XA and I made the change in ERAM.I advised the supervisor and asked if he wanted to take care of it or if I should; he said; 'go ahead'. I don't believe he took any action.Please share this information with whoever you see fit.  [Airline company] should be asked to advise their dispatchers to take greater caution when flight numbers involve different aircraft; especially at their HUBs. In this case; it wasn't a huge problem; it created confusion and additional workload; but it is nonetheless unacceptable.
A80 controller reports of two aircraft with the same call sign in the airspace; one an arrival and the other was a departure. Controller added a letter to the end of one of the calls signs.