Reason for duplicate keys on http post?

Reading about Http Post on Wikipedia it states that This is a format for encoding key-value pairs with possibly duplicate keys. Is this correct and if so what is the reasoning? Why would a client ever post duplicate keys and 开发者_JS百科if a duplicate key is posted how is the correct corresponding value returned on server side?

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To submit multiple values for the same thing.

In PHP, for example, you can name multiple input fields somedata[]. All values of the input boxes are then put in an array named somedata.