Duplicate "cannot open source file" appears when intelliSenseEngineFallback is "Disabled"

[sean-mcmanus]

1. Set "C_Cpp.intelliSenseEngineFallback": "Disabled"
2. Use
   #include <foo> #include <foo2>
   Requires at least 2 missing includes.

Bug: The errors appear twice. I found/fixed this while investigating another crash issue.

[sean-mcmanus]

Fixed with the preview .vsix at https://github.com/Microsoft/vscode-cpptools/releases/tag/v0.15.0-insiders (our final release is planned for next week).