How can I repeat the content of a file n times?

You don't need input-duplicated.txt.

Try:

mycommand <(perl -0777pe '$_=$_ x 1000' input-data.txt)

Explanation

• 0777 : -0 sets sets the input record separator (perl special variable $/ which is a newline by default). Setting this to a value greater than 0400 will cause Perl to slurp the entire input file into memory.
• pe : the -p means "print each input line after applying the script given by -e to it".
• $_=$_ x 1000 : $_ is the current input line. Since we're reading the entire file at once because of -0700, this means the entire file. The x 1000 will result in 1000 copies of the entire file being printed.

I was originally thinking that I would have to generate a secondary file but I could just loop the original file in Bash and use some redirection to make it appear as a file.

There are probably a dozen different ways of doing the loop but here are four:

mycommand <( seq 1000 | xargs -i -- cat input-data.txt )
mycommand <( for _ in {1..1000}; do cat input-data.txt; done )
mycommand <((for _ in {1..1000}; do echo input-data.txt; done) | xargs cat )
mycommand <(awk '{for(i=0; i<1000; i++)print}' input-data.txt)  #*

The third method there is improvised from maru's comment below and builds a big list of input filenames for cat. xargs will split this into as many arguments as the system will allow. It's much faster than n separate cats.

The awk way (inspired by terdon's answer) is probably the most optimised but it duplicates each line at a time. This may or may not suit a particular application, but it's lightning fast and efficient.

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But this is generating on the fly. Bash outputting is likely to be very much more slow than something can read so you should generate a new file for testing. Thankfully that's only a very simple extension:

(for _ in {1..1000}; do echo input-data.txt; done) | xargs cat > input-duplicated.txt
mycommand input-duplicated.txt

I would just use a text editor.

vi input-data.txt
gg (move cursor to the beginning of the file)
yG (yank til the end of the file)
G (move the cursor to the last line of the file)
999p (paste the yanked text 999 times)
:wq (save the file and exit)

If you absolutely need to do it via the command-line (this requires you to have vim installed, as vi doesn't have the :normal command), you could use:

vim -es -u NONE "+normal ggyGG999p" +wq input-data.txt

Here, -es (or -e -s) makes vim operate silently, so it shouldn't take over your terminal window, and -u NONE stops it from looking at your vimrc, which should make it run a little faster than it otherwise would (maybe much faster, if you use a lot of vim plugins).