How to find duplicate lines in a file?

I have an input file with foillowing data:

line1
line2
line3
begin
line5
line6
line7
end
line9
line1
line3

I am trying to find all the duplicate lines , I tried

sort filename | uniq -c  

but does not seem to be working for me :

It gives me :

  1 begin
  1 end
  1 line1
  1 line1
  1 line2
  1 line3
  1 line3
  1 line5
  1 line6
  1 line7
  1 line9

the question may seem duplicate as Find duplicate lines in a file and count how many time each line was duplicated? but nature of input data is different .

Please suggest .

use this:

sort filename | uniq -d
man uniq