Is the following sentence correct?
Confirm that someone is who they say they are
Not sure why "who they say they are" is used, as the antecedent to they is someone which is a singular pronoun and uses the singular form of be (is).
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This is an example of "singular they". Here it is being used, apparently, to avoid the need to use a gendered pronoun to refer ro a person of unknown or unspecified gender.
This is now a common usage. It has been used by respected writers of English since the 14th century, but is now much more common than it used to be. A few decades ago this sentence would have been likely rendered as :
Confirm that someone is who he says he is.