The general rule of thumb is that one should never write an article with only one source because journalists need to present a complete view of the issue - not just one side. Beyond that, anytime reporters want to publish something controversial, they are supposed to have three separate sources confirming the information. Those don't all have to be human sources - they may have one interview, one document, and their own eye witness account - but there are supposed to be three separate sources.
However, in reality, stuff like this happens a lot. Newspapers are first and foremost a business now. To those of us who work on the editorial side of newspapers (writers and editors) this has become a distressing reality, and the subject of much discussion and analysis. There are a number of excellent books and articles examining the problems that arise when publishers with no journalistic experience pressure newsrooms to cut costs while churning out stories that will entice readers and increase circulation - therefore increasing advertising revenue. When this happens, corners often are cut. Editors eager for a big scoop sometimes push through stories that should be held until the writer can get collaborating sources. Editors sometimes put too much trust in writers who are under a great deal of stress to produce "the next pulitzer." This is how things like the Janet Cook and Jayson Blair scandals happen. (Janet Cook was a Washington Post reporter who had her Pulitzer stripped after it was discovered that she had created a fictional child to profile by combining interviews with a number of different sources. Jayson Blair recently resigned from The New York Times after it was discovered that he plagerized many of his stories and in other cases invented sources and lied about interviews he claimed to have conducted.)
That was a long answer, and probably more than you wanted. The short version is that no, this is not acceptable, but unfortunately it's really rather unsuprising.
Annie
