There cannot be a common factor of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.
The factors of 18 are 1,2,3,6,9 and 18. For them to be common, they need to be compared to another set of factors.
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When the term 'common' is used we refer to two or more things. So "common factors of only one number" is an incorrect statement.