Several million people hit with new federal fines for going without health insurance will get a second chance to sign up starting Sunday, and that could ease the sting of rising penalties for being uninsured. But as the enrollment window reopens, it’s unclear how many know about the time-limited opportunity, let alone will take advantage of it. Fines payable to the IRS are the stick behind the offer of taxpayer-subsidized private insurance under President Barack Obama’s health care law. Virtually everyone in the country is now required to have coverage through an employer or a government program, or by buying...