How to duplicate lines in Linux with keeping the original order?

Question

I have a file (with only 1 column) like this:

A
B
Z
D
N

and what I want to do is to duplicate each line so I get this:

A
A
B
B
Z
Z
D
D
N
N

I only could think of using cat for the same file and then sort it:

cat file1 file1 | sort -k1 > file1_duplicate

but then I lose the order of my file which is important for me:

A
A
B
B
D
D
N
N
Z
Z

any suggestion would be helpful.

Answer 1

Try e.g.

sed p file >newfile