I have a couple thoughts, though I don't think they will make you certain of anything.I read that some of the drawbacks of the info on census reports were that (back then) the mobility of census takers was somewhat limited.One person would conduct the inquiries for an area, traveling from farm to farm until the district was complete. (It was so easy as to mail the family a letter and have them mail it back).Thusly, the census takers would often have to accept the information provided to them, by the person who was home on the day/time they went there. This means that they might obtain the information from a wife, a servant or a visiting relative.Often times the info was inaccurate, as the head of household or adult member of the family wasn't necessarily the one providing the info to the census taker.
As for two entries...Do you have the copies of the original handwritten census?If so, is the handwriting on the one entry different than the handwriting on the other?If so, perhaps the one census taker arrived and took info from one person and the other accidentally went to the same house and obtained info from another family member. Or, the first census taker couldn't complete the whole district, and the person hired to complete the job duplicated the effort (again, with a different family member reporting). Or perhaps if the handwriting is the same...the census taker lost the original record and repeated the visit and spoke with two different family members.
My guess is (and I'm certainly no expert) that this is the same family and they were visited twice (accidentally), with two different people reporting the info.
Look at the date the census was completed on the top of the census form...It may explain why Malinda was no longer in the household, as she was married March 1880.
Why was Jacob not listed in the census?He was born in 1878...He should have been listed with an age of 0 or 1.That's odd.