It means you should never rely on only one plan - in case it goes wrong.

这句话的意思是你不应该依赖唯一的计划——万一出错。

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In his speech Wednesday night, President Obama said the idea is only one part of his plan and should not be used as an excuse.

VOA: special.2009.09.12

Yet when one considers the need for affordable energy to power our homes and light our factories, only one plan stands a greater chance of success of securing our energy future.

FORBES: Which Candidate's Energy Plan is Better for America?

Forbes only offers one health plan.

FORBES: What's Going On During Open Enrollment At Your Company?

Worse, employers would only have access to one plan, vs. the wide array at the Massachusetts Connector.

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