Merge and remove redundant lines *among* files - r

Question

I need to merge several files, removing redundant lines among files, while keeping redundant lines within files. A schematic representation of my files is the following:
File1.txt
1
2
3
3
4
5
6
File2.txt
6
7
8
8
9
File3.txt
9
10
10
11
The desired output would be:
1
2
3
3
4
5
6
7
8
8
9
10
10
11
I would prefer to get a solution either in awk, or in bash or in R language. I searched the web for solutions and, though there were plenty of them* (please find some examples below), there were all removing duplicated lines regardless of the fact that they were located within or outside files.
Thanks in advance.
Arturo
Examples of previous solutions removing redundant lines both within and outside files:
https://unix.stackexchange.com/questions/50103/merge-two-lists-while-removing-duplicates
https://unix.stackexchange.com/questions/457320/combine-text-files-and-delete-duplicate-lines
https://unix.stackexchange.com/questions/350520/awk-combine-two-big-files-and-remove-duplicated-lines
https://unix.stackexchange.com/questions/257467/merging-2-files-and-keeping-the-one-duplicate

Answer 1

With your shown samples, could you please try following. This will NOT remove redundant lines within files but will remove them file wise.
awk '
FNR==1{
for(key in current){
total[key]
}
delete current
}
!($0 in total)
{
current[$0]
}
' file1.txt file2.txt file3.txt
Explanation: Adding detailed explanation for above.
awk ' ##Starting awk program from here.
FNR==1{ ##Checking condition if its first line(of each file) then do following.
for(key in current){ ##Traverse through current array here.
total[key] ##placing index of current array into total(for all files) one.
}
delete current ##Deleting current array here.
}
!($0 in total) ##If current line is NOT present in total then do following.
{
current[$0] ##Place current line into current array.
}
' file1.txt file2.txt file3.txt ##Mentioning Input_file names here.

Answer 2

Here's a trick adding on to https://stackoverflow.com/a/15385080/3358272 using diff and its output format. There is likely a presumption of "sorted" here, untested.
out=$(mktemp -p .)
tmpout=$(mktemp -p .)
trap 'rm -f "${out}" "${tmpout}"' EXIT
for F in ${#} ; do
{ cat "${out}" ;
diff --changed-group-format='%>' --unchanged-group-format='' "${out}" "${F}" ;
} > "${tmpout}"
mv "${tmpout}" "${out}"
done
cat "${out}"
Output:
$ ./question.sh F*
1
2
3
3
4
5
6
7
8
8
9
10
10
11
$ diff <(./question.sh F*) Output.txt
(Per markp-fuso's comment, if File3.txt had two 9s, this would preserve both.)