They, them and their with singular subjects

Take a look at the following citations:
If
a person is born of a gloomy temper they cannot help it.
It's enough to drive
anyone out of their senses.
Everyone, man and woman, has done their duty.
A spokesman said: 'A legal adviser has decided to leave over the past few days.' Asked the reason, he said: 'That is a matter for
them'

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