Car has trouble starting, especially after fueling

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So, first time: went to get gas, car wouldn't start, tried three times to get it started, finally pulled out, went to turn off engine to look at it, engine didn't stop for 5 seconds after I had the key out. Thought it was the ignition, tried the key much gentler, worked fine. After this, car runs perfectly. Fuel up #2: tried 5 times before car would start, once fuel dropped below full it was fine starting. Fuel up #3: almost 10 tries before it would start, once started had no problems. Each time, it sounded like the motor was starting, but wouldn't finish turning over. Only has happened when I fill up the tank so far...