Because they are: two or more atoms in the list of atoms have overlapping, or anyway too close, positions. Can't you see why? look better (or use XCrySDen: see above) and remember that the code checks periodic images as well.
Possible reasons: disk is full; outdir is not writable for any reason; you changed some parameter(s) in the input (like wf_collect, or the number of processors/pools) without doing a bit of cleanup in your temporary files; you were running more than one instance of pw.x in the same temporary directory with the same file names.
In most cases: you are treating a metallic system as if it were insulating.