In release versions of desktop applications, various issues (such as double login) may require implementation to prevent double startup.
In order to implement this feature, each developer need to modify the engine.
The engine already has code that uses NamedMutex to determine if it is the first instance.
It would be a great help to developers if this feature is provided as a project option.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-117812 in the post.