It is a term from another online "community" that furtive lured me into sometime last year. The D stands for double, the P stands for posting and I assume the M stands for moron. It (and its cousin DPW) gets used when someone posts a link that has already been posted.

In this case the definition was being stretched a little, as it was posted "at the other place" and then I cross posted it here.

Hope that clears it up