repeat string in a line using sed

Question

I would like to repeat each line's content of a file, any quick solution using sed.

supposed the input file is
abc
def
123

The expected ouput is:
abcabc
defdef
123123

Thank you for your help

Answer 1

sed 's \(.*\) \1\1 ' infile

Answer 2

sed 'h;G;s/\n//' file.txt