A RWY INCURSION OCCURS WHEN A PA28 CFI MOVES ONTO RWY FOR TKOF USING A CLRNC ISSUED TO A CESSNA FOR AN INTXN TKOF AT VRB; FL.

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DURING A BUSY MOMENT AT VRB; SEVERAL ACFT ON APCH AND HOLDING IN SEQUENCE TO DEPART RWY 4. I HAD THE CTLS ABOUT TO DEMONSTRATE SOFT FIELD TKOF PROCS TO A STUDENT PLT. WHEN WE GOT TO THE #1 POS; AND BEFORE I RPTED IN; CLRNC WAS GIVEN FOR IMMEDIATE TKOF. I HAD NOT RPTED POS YET DUE TO HVY RADIO SEQUENCING BY ATC AND I ASSUMED CLRNC WAS GIVEN IN THIS MANNER FOR PROMPT TFC FLOW. ACFT ON FINAL WAS VISUALLY VERIFIED FOR ENOUGH SEPARATION CLRNC; SO AS I ACCEPTED THE CLRNC AND ROLLED FOR THE RWY; AT THE SAME TIME; I NOTICED A CESSNA ROLLING FOR AN INTXN TKOF 1/2 WAY DOWN THE RWY. ATC ANNOUNCED THE INTXN TKOF CLRNC WAS GIVEN TO A CESSNA. I ANNOUNCED MY CONFUSION AS I PROMPTLY CONTINUED TO A TXWY DIRECTLY OPPOSITE ON THE OTHER SIDE OF THE DEP END. NO ACTION WAS NEEDED BY THE APCH TFC NOR INSTRUCTION GIVEN BY ATC TO DISRUPT THE ORIGINAL ONGOING FLOW. EVENTUALLY WE WERE CLRED THIRD FOR TKOF AFTER THE TFC THAT HAD BEEN IN SEQUENCE BEHIND ME; AND NOTHING WAS SAID BY ATC. I CANNOT SAY WHETHER THE CESSNA HAD A SIMILAR TAIL NUMBER OR THE CTLR USED MINE TO CLR IT. MY STUDENT ALSO BELIEVED IT WAS US WHO HAD BEEN CLRED; UNTIL CORRECTIVE ACTION WAS TAKEN. THE CESSNA WAS THE ONLY OTHER TFC FOR AN INTXN DEP AND I WAS NOT AWARE OF ITS PRESENCE. FACTORS WERE: THE HVY TFC AT THE TIME; RUSH TO ACCEPT A CLRNC BEFORE VERIFYING AND LACK OF AWARENESS OF OTHER TFC HOLDING FOR INTXN DEP ON THE SAME RWY; VISUALLY BECAUSE I WAS PREOCCUPIED WITH THE APCHING TFC AND AURALLY BECAUSE I HAD SWITCHED TO TWR FREQ BUT ONLY A FEW MOMENTS BEFORE.