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Rental car company Hertz Global Holdings has adopted a one-year shareholder rights plan because of what it calls ‘‘unusual and substantial activity’’ involving its shares. Hertz, which is based in New Jersey and has dozens of rental locations in Massachusetts, said the plan often referred to as a ‘‘poison pill’’ will take effect if a person or group acquires a 10 percent stake in the company. Or the plan could go forward if a passive institutional investor acquires a 15 percent stake.