You don't go to the store and buy an egg - you buy a dozen.  Contained within that dozen are the individual eggs. 

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My guess:

 You can´t go to store and buy eggs in units, only in dozen. (In this case, it´s a redundancy to say that a dozen has ''individual egg'', even though "individual" is proper to talk about "indivíduo" (Portuguese) a person; more often used in, say, sociological texts.