Typescript guide gives "Duplicate function implementation" warning

This might be because you don't have a tsconfig.json file for your TypeScript project. Try creating a tsconfig file and write a default "compilerOptions". It worked for me. The tsconfig.json file with default code that I used is :

{
    "compilerOptions": {
        "module": "commonjs"
    },
    "exclude": [
        "node_modules"
    ]
}

For more info on VS TypeScript Compiling please refer https://code.visualstudio.com/docs/typescript/typescript-compiling

If you have both the src file (typescript) and the transpiled file (javascript) in the same directory and open up the javascript file in VS Code then you'll get the error. Output the transpiled file into a directory and it will not error. Use the --outDir flag: tsc --outDir ./dist greeter.ts

Got this problem in version 1.26.1 of VS Code. Generating the tsconfig.json file did not make the error go away for me.

Looks like this is a bug in Visual Studio Code. There are a few issues on GitHub about this, such as here and here. The comments on the issues imply that it was an issue, then was fixed, and has just become an issue again in v1.12.1.

It looks as if the solution is to run tsc --init to initialize the tsconfig.json in the folder.

When we open both file.ts and the transpiled file.js files and do TSC, this error occurs.
Please close the transpiled file.js and try again.