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I have a number of lists (only 3 shown here)

 List1 = {{1, 2}, {1, 2, 3}, {1, 2, 3, 4, 5}};
 List2 = {{1, 2, 4, 8, 7}, {1, 2, 9, 10}};
 List3 = {{9, 11, 1}, {1, 9, 11, 13}, {1, 9, 12}, {1, 9, 14}};

I want to combine these lists in the following way: Union the last element of List1 with each element of List2, and add it to the end of List1, then Union the last element of List2 to all the elements of List3 and add them to the end of List1.

Producing

{{1, 2}, {1, 2, 3}, {1, 2, 3, 4, 5}, {1, 2, 3, 4, 5, 7, 8}, {1, 2, 3, 
 4, 5, 7, 8, 9, 10}, {1, 2, 3, 4, 5, 7, 8, 9, 10, 11}, {1, 2, 3, 4, 
 5, 7, 8, 9, 10, 11, 13}, {1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 
 13}, {1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13}}

I should mention, the numbers are random, although here it appears as consecutive at some places.

Edit

With @ciao's help, I have the solution

Union@FoldList[Union, Join[List1, List2, List3]]

However, it seems to drop repeated elements as it appears in List3.

Edit 2

Fidgeting around for a bit, I realized on dropping the outer Union, the problem is solved.

FoldList[Union, Join[List1, List2, List3]]
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    $\begingroup$ You could have devised a perhaps shorter and more clear example! $\endgroup$
    – Dr. belisarius
    Jun 26, 2015 at 22:50
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    $\begingroup$ Your output does not match what you describe. r1 = Union[list1[[-1]], #] & /@ list2; r2 = Union[list2[[-1]], #] & /@ list3; r3 = {r1, r2} $\endgroup$
    – Nasser
    Jun 26, 2015 at 23:08
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    $\begingroup$ Union@FoldList[Union, Join[list1, list2, list3]] produces the output shown... but that's about as far as I'm willing to decode the OP - perhaps a clearer example/explanation is warranted? $\endgroup$
    – ciao
    Jun 26, 2015 at 23:26
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    $\begingroup$ @ciao Is there a way to modify your code so that it doesn't truncates identical elements in the list? See edited version. $\endgroup$
    – HuShu
    Jun 27, 2015 at 0:42
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    $\begingroup$ @HuShu: +1 on figuring out your question to me... $\endgroup$
    – ciao
    Jun 27, 2015 at 6:27

1 Answer 1

Reset to default
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This works

   FoldList[Union, Join[List1, List2, List3]]
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