“一次且仅一次”是极限编程的座右铭。
传输可靠性有类似的性能问题，但是其令人感兴趣的好处是一次且仅一次传送。
Transport reliability has similar performance concerns but the interesting benefit is once-and-only-once delivery.
比尔：你在《重构》一书中提到：“代码应该实现任何逻辑一次且仅一次，这是良好设计的要素”。
Bill Venners: in your book, Refactoring you say, "the code says everything once and only once, which is the essence of good design."
本题中周先生想到各地旅游，每个城市都被访问一次且仅一次，我们考虑到了旅行商问题的解决方法。
The problem in Chow expect travel around, every city is accessed once and only once, we take into account the solution to the traveling salesman problem.
这里没有使用一次且仅一次（once-and-only-once）或者重试直至成功（retry-until-success）的语义以允许可靠传输HTTP请求的机制。
There are simply no mechanisms built in that allow HTTP requests to be reliably delivered using once-and-only-once or retry-until-success semantics.
如果流程要按照正常方式执行，则接收或选择活动必须执行一次，且仅执行一次。
A receive or pick activity must be executed once and only once if a process is to be executed normally.
它也确保依赖项已经加载一次，且仅加载一次。
It also makes sure dependencies are loaded once and only once.
它也确保依赖项已经加载一次，且仅加载一次。
It also makes sure dependencies are loaded once and only once.
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