He let himself in with a duplicate key.

他用一把另配的钥匙打开门进去了。

First have a duplicate key cut.

你先去配把钥匙。

May I borrow a duplicate key?

我可以借用备用钥匙吗？

May I Borrow a duplicate key, please?

请把备用钥匙借给我好吗？

Which means no duplicate key is allowed.

这意味着没有重复的关键是允许的。