X LANDED ON OCCUPIED RWY OVER THE TOP OF Y HOLDING IN POS. SYS ERROR.

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I WAS WORKING LCL CTL AT THE VFR TWR. BOTH OF OUR LCL CTL POS WAS COMBINED AT 1 LCL POS/SECTOR. WX WAS VFR. TWR EQUIP AVAILABLE: BRITE RADAR DISPLAY. THE TFC WAS MODERATE TO HVY. EXPERIMENTAL ACFT X IN QUESTION WAS GIVEN INSTRUCTIONS TO MAKE STRAIGHT-IN RWY 9L. LATER I PUT A CESSNA C-172 Y INTO POS AND HOLD ON RWY 9L. I PUT THAT C-172 INTO POS AND GAVE THE EXPERIMENTAL A CLRNC TO LAND BASED ON A TARGET I SAW ON A 2 MI FINAL AS DISPLAYED ON THE BRITE RADAR. THE ACFT ON THE 2 MI FINAL TURNED OUT NOT TO BE THE EXPERIMENTAL. INSTEAD; THE EXPERIMENTAL WAS XING OVER THE THRESHOLD RWY 9L ABOUT 150 FT OVER THE C- 172. I DON'T KNOW HOW FAR TO THE L OR R THE EXPERIMENTAL WENT AROUND THE C-172. NO ACCIDENT JUST A POSSIBLE LOSS OF SEPARATION. CONTRIBUTING FACTORS: TFC LOAD AND SECTORS COMBINED TOO MUCH FOR CTLR AND MOST IMPORTANT -- BRITE RADAR DISPLAY UNRELIABLE BY NOT ALWAYS DISPLAYING ALL TARGETS IN THE AIRSPACE. (THIS IS A RECURRING PROB THAT HAS BEEN BROUGH TO MGMNT'S ATTN MANY TIMES.) I NEVER SAW A RADAR TARGET DISPLAYED FOR THE SMALL EXPERIMENTAL AND MISTOOK ANOTHER ACFT'S TARGET WHILE BEING VERY BUSY.