ACR X POSSIBLE LNDG ON OCCUPIED RWY POSSIBLY HAD LTSS FROM ACR Y. POSSIBLE SYS ERROR PLTDEV.

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ACR X NIGHT TIME LNDG WITH VISIBILITY 4 MI IN LIGHT RAIN AND FOG. CLRED TO LAND BEHIND ACR Y ACFT; WHICH BOTH CAPT AND I HAD IN SIGHT. RWY IS 9500 FT LONG. ACR Y USED FULL LENGTH. ACR X WAS APPROX 100 FT AGL; WHEN ACR Y WAS BEGINNING TO TURN OFF AT THE END. IT APPEARED THAT ACR Y WAS GOING TO CLR IN TIME; SO THE CAPT (PF) CALLED FOR LNDG FLAPS AND THE FINAL CHKLIST. JUST AS HE DID; THE TWR CTLR ONCE AGAIN CLRED US TO LAND. WE LANDED; TURNED OFF AT MIDFIELD; AND TAXIED TO OUR GATE. LATER; IN OPS; WE WERE APCHED BY A FSDO INSPECTOR WHO DEMANDED INSPECTION OF OUR MEDICALS AND LICENSES. HE TOLD THE CAPT AND ME THAT HE HAD BEEN ON ACR Y AND IN HIS OPINION WE HAD LANDED ON THE RWY BEFORE ACR Y HAD COMPLETELY CLRED. IN OUR OPINION; WE MOST CERTAINLY DID NOT (AND WOULD NOT) LAND ON AN OCCUPIED RWY. APPARENTLY; THE TWR CTLR (BY AGAIN CLRING US TO LAND) FELT THE SAME. SUPPLEMENTAL INFO FROM ACN 214815: ACR Y WHILE LNDG ON RWY 13 AT MDT; MDT TWR CLRED ACR X TO LAND WHILE WE WERE STILL ON THE RWY SLOWING TO A SAFE TAXI SPD (TOWARD TAXIWAY E). AS WE ROLLED OFF RWY 13 ON TAXIWAY E THE ACR X WAS ROLLING OFF AT TAXIWAY C. THE OTHER ACFT HAD LANDED AND SLOWED WHILE WE WERE STILL ON THE RWY -- WITH ATC'S APPROVAL. UNFORTUNATELY; AN FAA INSPECTOR WAS ON BOARD OUR FLT AND SAW THE WHOLE THING. HE ASKED FOR OUR CERTIFICATES AND ASKED IF WE SAW THE OTHER ACFT LAND WHILE WE WERE STILL HALFWAY DOWN THE RWY; GOING TOWARD TAXIWAY E. WE GAVE OUR RENDITION AND THE FAA GUY GOT OFF AND TOLD US WE WOULD BE HEARING FROM HIM. THE PROBLEM LIES ON THE TWR AT MDT. THIS IS NOT THE FIRST TIME THIS HAS HAPPENED. THE PLANES ARE SEQUENCED MUCH TOO CLOSE AND MOST OF THE 'LCLS' WON'T DO A GAR BECAUSE TWR ALLOWS IT. SOMETHING NEEDS TO BE DONE AS SOON AS POSSIBLE ABOUT THIS PROBLEM.