Date: 201204
Local time of day: 1801-2400
Location: ZSU.ARTCC, PR
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
Airspace: Class A ZSU
ATC / advisory: Center ZSU
Second aircraft: A330
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness; Confusion
Second reporter function: First Officer; Pilot Flying
Anomaly: ATC Issue All Types
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Similar sounding call signs are going to cause a serious accident if action is not taken. I estimate 20 plus aircraft in my sector during the time of incident. The sector should have been de-combined but the FLM was not engaged. I issued a descent clearance to Air Carrier ZXX from FL330 to FL310 to get them under the A330 who was descending to FL320 landing MDPC. Opposite direction to Air Carrier ZXX was Air Carrier XXX at FL340 who took the descent clearance for Air Carrier ZXX; FL340 descending to FL310. So Air Carrier ZXX is still at FL330 with the A330 converging and descending through FL340 for FL320. It was too late to stop the descent of the A330. I screamed at Air Carrier ZXX to begin the descent thinking the pilot thought it was a PD. Air Carrier XXX heard this and realized what was going on meanwhile he was opposite to Air Carrier ZXX. Now these two were both descending to FL310. Air Carrier ZXX barely got under the A330 in time and now Air Carrier XXX started climbing back up to FL340. Air Carrier XXX said I had told them initially to descend to FL310 and they had read it back. Regardless who did what; that was very close and unsafe and was a direct result of this similar sounding call sign madness that plagues our industry. Please make this situation a top priority to mitigate. ZSU has the same call signs everyday and it is reported dozens of times and nothing happens.


Second narrative: During flight we were advised by San Juan Center that Company Flight ZXX was on the same frequency. We acknowledged; as did Flight ZXX. Approximately 5 minutes after level off at FL340; ATC issued a descent to FL310 for traffic to Flight XXX (our callsign). We acknowledged and used our callsign and begun a descent to FL310. About three minutes later; as we were leveling at FL310; we heard ATC question Flight ZXX about why they had not started their descent. They said they had not been given a clearance and were immediately directed to descend and turn for traffic. I realized that the Controller had given the descent to us instead of Flight ZXX. I questioned ATC as to the altitude he wanted us at and pointed out that he had given Flight XXX the descent not Flight ZXX. He immediately told us to return to FL340 and expedite through FL320. The Controller got confused by the similarity in the call signs. Maybe we could investigate our flight numbers and routes to try to reduce the number of similar call signs in close proximity to each other.


Synopsis: ZSU Controller voiced concern regarding the continued use of similar sounding call signs by aircraft operating within their airspace citing a recent conflict resulting from similar sounding call signs.