Narrative:

This was a red eye to fll. It was dark outside and the radio was pretty quiet with very sporadic transmissions. As we approached fll we checked in with miami center and they acknowledged us checking in at FL350. Approximately 230 miles from fll while at FL350 we got a call from miami center asking us to descend to FL200. I answered the call with 'descend and maintain flight level two zero zero; air carrier XXX' the first officer; who was the flying pilot; set 20;000 in the altitude window and started a gradual descent. At the time I thought we were still a little too far from fll to start our descent but because of traffic going into miami I have had them issue this early descent in the past so I thought nothing of it. As we descended through FL260 miami center issues a clearance for us to descend at pilot's discretion to FL240. I tried to figure out what was going on and after a few seconds the miami center controller re-issued the clearance for us to descend at pilot's discretion to FL240. I answered the controller by saying that I was trying to figure out the last clearance that had already been issued us a previous clearance for us to descend and maintain FL200. The controller then informed me that they were manning two frequency's and that there was another aircraft with a similar callsign and same XXX flight number on the controller's other frequency. This was the first time we had heard anything regarding the other aircraft with similar callsign and same flight number. The controller then said for us to be aware of the other aircraft on the other frequency with a similar callsign and same XXX flight number and re-issued the clearance for us to descend at pilots discretion to FL240. I read back the clearance and continued descending from FL260 to FL240. The controller then advised the other aircraft of our similar callsign and same XXX flight number. The radio was very quiet but we never heard the other aircraft check in so I have to assume that they had checked in with miami center before we did. There were no TCAS alerts or other indication that there had been a conflict with another aircraft but the fact remains that we descended 9;000 before the controller issued the clearance for us to descend at pilot's discretion. Nothing else was said by the controller. Cause; two aircraft with similar callsigns and exact same flight number in the same sector being handled by the same controller in two different frequencies and neither one of the aircraft was aware of the other. Neither aircraft had the chance to catch the error by saying 'I think somebody else took our clearance'. The controller should have also heard two read backs as he/she was manning both frequencies. Suggestions; by simply making sure each aircraft were aware of each other and for the controller to issue very clear instructions of which aircraft needs to do what.

Google
 

Original NASA ASRS Text

Title: Air Carrier on frequency with ZMA described callsign confusion when the Controller was working two frequencies with two aircraft with identical flight numbers; no advisory was issued by ATC.

Narrative: This was a red eye to FLL. It was dark outside and the radio was pretty quiet with very sporadic transmissions. As we approached FLL we checked in with Miami Center and they acknowledged us checking in at FL350. Approximately 230 miles from FLL while at FL350 we got a call from Miami Center asking us to descend to FL200. I answered the call with 'Descend and maintain Flight level Two Zero Zero; Air Carrier XXX' The First Officer; who was the flying pilot; set 20;000 in the ALT window and started a gradual descent. At the time I thought we were still a little too far from FLL to start our descent but because of traffic going into Miami I have had them issue this early descent in the past so I thought nothing of it. As we descended through FL260 Miami Center issues a clearance for us to descend at pilot's discretion to FL240. I tried to figure out what was going on and after a few seconds the Miami Center Controller re-issued the clearance for us to descend at pilot's discretion to FL240. I answered the Controller by saying that I was trying to figure out the last clearance that had already been issued us a previous clearance for us to descend and maintain FL200. The Controller then informed me that they were manning two frequency's and that there was another aircraft with a similar callsign and same XXX flight number on the controller's other frequency. This was the first time we had heard anything regarding the other aircraft with similar callsign and same flight number. The Controller then said for us to be aware of the other aircraft on the other frequency with a similar callsign and same XXX flight number and re-issued the clearance for us to descend at pilots discretion to FL240. I read back the clearance and continued descending from FL260 to FL240. The Controller then advised the other aircraft of our similar callsign and same XXX flight number. The radio was very quiet but we never heard the other aircraft check in so I have to assume that they had checked in with Miami Center before we did. There were no TCAS alerts or other indication that there had been a conflict with another aircraft but the fact remains that we descended 9;000 before the Controller issued the clearance for us to descend at pilot's discretion. Nothing else was said by the Controller. Cause; two aircraft with similar callsigns and exact same flight number in the same sector being handled by the same controller in two different frequencies and neither one of the aircraft was aware of the other. Neither aircraft had the chance to catch the error by saying 'I think somebody else took our clearance'. The Controller should have also heard two read backs as he/she was manning both frequencies. Suggestions; by simply making sure each aircraft were aware of each other and for the controller to issue very clear instructions of which aircraft needs to do what.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.