TWR FORGOT THAT IT HAD AN ACFT HOLDING IN POS FOR TKOF AND CLRED THE RPTR TO LAND; WHICH IT DID.

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APCHING SAN JOSE ARPT; APPROX 15-20 MI OUT; WE WERE CLRED FOR THE VISUAL APCH. THE CAPT WAS FLYING; I WAS HANDLING THE RADIO AND CHKLISTS. AT 8-10 MI OUT WE WERE HANDED OFF TO SAN JOSE TWR AND WERE CLRED TO LAND. AS I COMPLETED THE LAST ITEMS OF THE BEFORE LNDG CHKLIST WE NOTICED A BLINKING LIGHT (POSSIBLY AN ACFT OR GND VEHICLE) AT THE BEGINNING OF THE DISPLACED THRESHOLD OF RWY 30L (OUR RWY). AT THIS POINT WE WERE APPROX 3 MI FROM THE RWY. THE TWR CTLR WAS BUSY WITH TKOF AND LNDG INSTRUCTIONS FOR ACFT USING THE PARALLEL RWYS 29 AND RWY 30R WHICH BLOCKED OUR ATTEMPTS TO VERIFY LNDG CLRNC. ON 1/2 MI FINAL; WHILE STILL TRYING TO DETERMINE WHAT WE WERE SEEING (ACFT; VEHICLE; OR WHAT?) ON THE DISPLACED THRESHOLD; I WAS FINALLY ABLE TO RECONTACT THE TWR AND VERIFY THAT WE WERE 'CLRED TO LAND.' STILL UNSURE OF A POSSIBLE CONFLICT; THE CAPT SIDE-STEPPED SLIGHTLY TO THE L OF CTRLINE AND AS WE FLEW OVER THE DISPLACED THRESHOLD (1500 FT FROM THE NUMBERS) I LOOKED DOWN AND COULD SEE AN ACFT HOLDING IN POS. WE LANDED WITHOUT INCIDENT; AND AFTER PARKING THE CAPT CONTACTED THE TWR SUPVR BY PHONE TO DISCUSS THE SIT. THE SUPVR TOLD THE CAPT THAT THE TWR CTLR HAD FORGOT THAT ANOTHER ACFT HAD BEEN CLRED INTO POS AND ACCEPTED RESPONSIBILITY FOR THE ERROR. IN DISCUSSING THE SIT SHORTLY THEREAFTER THE CAPT AND I BOTH AGREED THAT A GAR WOULD HAVE BEEN A WISER CHOICE OF ACTION EVEN THOUGH THE LNDG PORTION OF THE RWY WAS CLR AND WE HAD BEEN 'CLRED TO LAND.' CONTRIBUTING FACTORS: NEITHER THE CAPT NOR I COULD RECALL HEARING THE TWR CLR AN ACFT INTO POS; THOUGH THIS MAY HAVE OCCURRED PRIOR TO OUR HDOF TO THE TWR. AS IT WAS DUSK AT OUR TIME OF LNDG; THE AMBIENT LIGHT WAS INSUFFICIENT TO SEE THE FUSELAGE OF AN ACFT ON THE DISPLACED THRESHOLD; YET NOT YET DARK ENOUGH FOR THAT ACFT'S POS LIGHTS AND ROTATING BEACON TO BE EASILY DISTINGUISHABLE FROM THE RWY LIGHTS.