Duplicate identifier error messages in Visual Studio projects when TypeScript files are moved from one location to another in Solution Explorer

A quick note from the field: if you have a web application project with TypeScript files in Visual Studio 2017, you might start to get error messages referring to "Duplicate identifier ‘XXX'" if you move TypeScript files from one location to another in Solution Explorer.

The new TypeScript project system does not need you to list available TypeScript files in your project. Instead, Visual Studio automatically scans for the files on disk. However, in the case of a move operation in Solution Explorer, Visual Studio does not (yet) keep up, and you might get multiple error messages immediately after the move operation.

The remedy is to close the project/solution, and open it again.