CESSNA C208 PLT HAS CABIN DOOR OPEN DURING TKOF.

Narrative

I TAXIED TO RWY 21 AT INTXN F; AND CONTACTED SPOKANE TWR. THE WIND WAS APPROX 250 DEGS AT 20 KTS GUSTING TO 30 KTS. AFTER RECEIVING TKOF CLRNC; I ROLLED ONTO THE RWY AND BEGAN MY TKOF. JUST BEFORE OR DURING LIFTOFF THE DOOR WARNING LIGHT CAME ON. I TOUCHED DOWN AND STOPPED WHILE ADVISING TWR OF THE SITUATION. I TAXIED OFF AT INTXN C; AND GOT PERMISSION TO GET OUT AND CLOSE THE DOOR. AT THIS POINT I THOUGHT THAT I HAD FORGOTTEN TO LATCH THE DOOR. (WHEN I PREFLTED THE ACFT; I UNLATCHED THE DOOR TO LET THE FREIGHT HANDLERS KNOW THAT IT WAS UNLOCKED; BUT DID NOT OPEN IT SINCE THE WIND WAS SO STRONG. ALL OF THE FREIGHT GOT LOADED IN THE POD; AND THE DOOR WAS NEVER OPENED. BECAUSE OF THIS I REASONED THAT I HAD NOT LATCHED THE DOOR AGAIN BEFORE DEPARTING.) I CLOSED THE DOOR; GOT BACK IN AND GOT CLRNC TO TAXI BACK TO INTXN E FOR DEP. (INTXN E IS APPROX 1/3 OF THE WAY DOWN RWY 21; WHILE TXWY C IS APPROX 2/3 OF THE WAY DOWN THE RWY.) I AGAIN RECEIVED TKOF CLRNC; TOOK OFF; AND AT ABOUT 50 FT AGL THE LIGHT CAME ON AGAIN. I LOOKED OVER MY L SHOULDER ALONG THE INSIDE OF THE EMPTY CABIN; AND I COULD SEE THAT THE DOOR WAS OPEN ABOUT 6 INCHES. I LOWERED FULL FLAPS PER THE CESSNA CHKLIST TO KEEP THE DOOR CLOSED; AND ADVISED THE TWR. I ELECTED TO CONTINUE AROUND THE PATTERN RATHER THAN LAND ON THE FAR END OF RWY 21 AND RISK HAVING THE CARGO DOOR DAMAGED BY BLOWING OPEN AS I TURNED BROADSIDE TO THE WIND WHILE TAXIING OFF OF THE RWY.