When I generate a Word doc in single-source layout, some of the headings in the TOC get listed twice. This appears to be random; the duplicated headings are listed only once in the TOC as it appears in RoboHelp, just like the others. I tried deleting and reconstructing the TOC but that didn't help and I'm not sure what else to do.
I don't know if this is what you're experiencing, but I do know that if a RH topic is linked to a book in the table of contents, the generated Word document will include both the title of the book and the title of the topic. That is, if the Robohelp TOC includes a book called "Getting Started" that links to a topic called "Getting Started", Word will include two instances of the "Getting Started" heading.
To get around this, I created a conditional build tag called "Online". When I create Books with Links in the table of contents, I manually apply this tag to the heading in the topic. I then generate the Word document with a "NOT Online" Conditional Build Expression.
Hope this helps,