AN ACR MLG MADE A GAR WHEN TCASII AND AN ACFT SHADOW COMMANDED IT.

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ON APCH INTO DEN; WE WERE APCHING THE ARPT FROM THE SE. WE HAD BEEN HANDED OVER TO THE FINAL APCH CTLR. THIS CTLR POINTED OUT 2 ACFT; AN ACR MLG AND A COMMUTER MDT. THE MDT WAS LNDG ON RWY 26R; THE MLG ON RWY 26L. WE ACKNOWLEDGED WE HAD BOTH ACFT IN SIGHT; BUT THAT WE NEEDED TO S-TURN IN ORDER TO GAIN SOME SEPARATION ON THE MLG. THE CTLR ADVISED US THAT WAS APPROVED; BUT TO S-TURN TO THE S OF FINAL FOR RWY 26L; CLRED VISUAL APCH RWY 26L BEHIND THE MLG; CONTACT TWR. THE APCH WAS IN GOOD WX; BUT THERE WAS HVY TFC INTO DEN. THE APCH WAS GOING WELL; WE HAD S-TURNED AND GAINED THE SPACING WE NEEDED. WE HAD STABILIZED BACK ON THE ILS FOR RWY 26L AND CONTINUED. AT THIS POINT WE WERE CLRED TO LAND BY TWR ON RWY 26L. EVERYTHING LOOKED FINE; UNTIL THE TCASII ALERTED US TO AN ACFT CLOSING ON US FROM BEHIND AND UNDERNEATH US SAYING 'CLB; CLB.' AT THIS MOMENT; I ASKED TWR; 'DID HE HAVE ANY TFC UNDER US?' HE SAID 'NEGATIVE.' THE CAPT WAS FLYING; SO I LOOKED OUT MY R WINDOW TO SEE IF I COULD VERIFY THE TCASII ALERT. SURE ENOUGH; I SAW WHAT APPEARED TO BE THE SHADOW OF AN SMT CONVERGING ON US. I COULD NOT SEE THE ACFT; JUST THEIR SHADOW. THE CAPT WAS IN THE PROCESS OF EXECUTING A GAR WHEN I TOLD HIM WHAT I SAW. SHORTLY AFTER THAT; THE TCASII COMMANDED A MORE URGENT CLB RESPONSE. IN THE PROCESS OF THE GAR; WE WERE UNABLE TO DETERMINE IF ANOTHER ACFT HAD LANDED ON RWY 26L OR RWY 26R. ON THE GND THE CAPT CALLED THE TWR BY PHONE TO SEE WHAT HAPPENED. THE TWR QUICKLY TRANSFERRED HIM TO THE APCH SUPVR. HE DID NOT HAVE ANY ANSWERS FOR US. ALL TURNED OUT WELL; BUT WHAT BOTHERED ME WAS HOW CLOSE THE ACFT WERE BEING VECTORED IN. THEN TO GET A TCASII COMMAND TO CLB WHEN THE TWR SAYS THERE IS NOBODY THERE; BUT I SEE A SHADOW; JUST UNSETTLES ME A BIT.