Xcode build error - Multiple commands produce .o, Target 'ProjectCoreData' has compile command for Swift source files listed twice

The reason you see this error is because Core Data creates these files by default, but you can't see them in Project Navigator. Then you create a duplicate (in Editor/Create NSManagedObject Subclass). As a result you have your error.

According to What's new in Core Data here is a picture with the solution:

For future users : If you are using core data and If you copied entity using option+drag, then ensure that entity name and class name must be same. None of the solutions mentioned over internet worked for me.