can you say that something or someone is the favorite if it's one of its kind?

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example: a woman with an only child saying her child is her favorite child is not incorrect

some might say this is a comparison that needs two separate elements with similarities and differences which is not the case here since there is only one child and therefor it's incorrect.

for me correctness isn't even a factor here. instead, it's usefulness that matters. language is a limited vessel of what we think and if the phrase "my only child is my favorite child" conveys the thoughts of the mother it is useful enough to be "correct"