A CRJ90 Crew reported a Position and Hold clearance issued for their flight number and they taxied into position. They were then told the clearance was for another aircraft with the same flight number but a different carrier.

Narrative

ATC had cleared us to taxi to Runway 9 via Taxiways B and M. While at the Runway 9 hold short line facing northeast on Taxiway M; the First Officer and I both heard ATC clear our Company and flight number cleared into position and hold Runway 9. The First Officer read back the clearance while I moved the aircraft forward across the hold line and then turned to line up on Runway 9. As I turned to line up with the runway; I heard Tower instruct another air carrier with the same flight number to hold short Runway 9. It then appeared that ATC had originally cleared the other flight to taxi into position and hold on Runway 9. ATC then cautioned us about similar sounding call signs and referenced the other air carrier. At the time when we accepted the position and hold clearance; we had never heard the other aircraft's callsign; and that aircraft was still taxiing southwest bound on Taxiway M toward the other Runway 9 hold line. At the time the First officer and I heard ATC clear us into position and hold Runway 9; our aircraft was stopped at the hold line with the parking brake set and with all normal checklists completed to that point. The cockpit was sterile and we were sitting quietly waiting our turn to go. We had no prior knowledge of the other flight with the similar callsign; and there were no other aircraft in sequence at Runway 9. I believe there was an expectation bias that we would be next to go coupled with the unknown similar sounding callsign which led to our mistake. It would have been helpful to have prior knowledge of the the similar sounding callsign.