It sounds like something out of Weekly World News, but it's actually straight out of a report in the Hong Kong Medical Journal: A baby girl was found to be carrying two masses which could be partially developed fetuses inside her abdomen. The extremely rare phenomenon is called fetus in fetu (literally "fetus inside fetus"), and doctors are still puzzled by it.
ABC News reports that doctors suspected the otherwise healthy baby girl of having a tumor in her left side. And the two masses they removed from the space between her liver and left kidney could be just that. While both reportedly contained spines, intestines, bones, primitive brain matter, and umbilical cords, that doesn't necessarily mean they were "babies".
It's possible that the two masses, which have the appearance of fetuses at about 10 weeks of gestation, are actually tumors called teratomas. A teratoma is made up of more than one kind of cell type, so it can end up full of structures like teeth and hair.
But while it's impossible that the baby girl was actually pregnant, it is possible that the masses inside her were once her siblings. Fetus in fetu occurs when a twin (or in this case, two triplets) are somehow "absorbed" by the surviving baby. Just how this absorption occurs remains unclear, because most known cases aren't discovered until after the surviving baby is born.
It's much more common for a twin to "vanish" early in the pregnancy than to reappear as a parasitic growth inside their sibling, but doctors aren't sure why this sometimes goes wrong. It's an eerie prospect, to be sure, and crops up as a plot-line in horror stories every now and again. But it's been used as a joke to great effect in movies like "My Big Fat Greek Wedding":
The Hong Kong baby is lucky to have had her tumors (or twins) removed early and without complication. In 1999, doctors removed a large, malformed twin that had lived inside its brother's abdomen for 36 years, causing him breathing pain and embarrassment.