Reading about association rules mining made me somehow confused. I am wondering if this is possible that in a transaction, an item occurs more than once? Considering the following database as a reference:

Transaction id  Items
         t1     {1,4,1}
         t2     {2}
         t3     {1,2,1}
         t4     {2,2}
         t5     {1, 2, 3, 5}