mergeList 

this function can avoid to copy element  that occurs more than once, but not if the elements are on the same list. es:
first list:
1 2 3 7 8 9
second list:
1 3 5 6 8
merged list:
12356789 (  no repeated element)
______________________________________
first list: 
2 3 5
second list:
1 3 3 8 9
merged list:
1 2 3 3 8 9 (repeated element)