Eric Perkins believes Tesla Inc. shares have climbed too quickly, to heights the company’s earnings can’t sustain. He’s still not selling.

A 44-year-old living outside San Diego, Mr. Perkins says he normally would sell to lock in a profit when a stock quadruples in four months. But after missing past Tesla rallies, he is determined to hold onto his shares, which now make up about half of his roughly $1 million portfolio.

“I…

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