bug#24244: bug: dd deletes file

[puggy]

i've just found a bug in "dd".

by mistakes i used the same input and output file.
dd said it wrote 0 bytes, but what it actually did
is overwrite the file and setting it back to zero.
doing so, dd deleted a 4.3G file in a fraction of
a blink.

luckily i can download the file again. but before 
it took me 2 hours to download this file. now i 
have to download it again, wait for another 2 
hours and i can't work for this time. 2 hours for
a little mistake. 4 hours time loss!

this can't happen. mistakes happen. by simple
logics it doesn't make sense to delete a file,
just because input and output file are the same.
also there is barely anything easier to check
for.

please find some solution for it. if it is
intentional, make an option for it. this way
you can also shorten the command. otherwise 
if input and output are the same, warn the user!