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In words, A is a subset of B if every element of A is also an element of B. (Some people write rather than ⊂.)
Since a set is just a collection of objects {1, 2, 3} and {1, 1, 2, 2, 3, 3} are both sets but by the definition of equality they are the same set, that is {1, 2, 3} = {1, 1, 2, 2, 3, 3}. The convention is to write this set {1, 2, 3} since listing elements more than once is redundant. I hope this helps,
A set does not have duplicate elements but a multiset does. In a multiset the number of times an element appears is important. Penny  


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