Printer feeds two sheets at once

My HP D110 occasionally and randomly feeds two sheets at a time. Sometimes it feeds just fine for 20 or more page, and sometimes it feeds multiple sheets as often as every every 3rd or 4th page.

The 2nd sheet lags behind the first by anywhere from 1/2 inch up to 3 or 4 inches. This requires reprinting both pages, and is quite a nuisance.

Any ideas?