Windows Attach API client creates named pipe to communicate with target VM (CreateNamedPipe).
It makes sense to update the pipe mode:
Add FILE_FLAG_FIRST_PIPE_INSTANCE to the open mode.
The pipe is created with max instances argument == 1, so if another clients tries to create a pipe with the same name, it fails with ERROR_PIPE_BUSY.
But max instances value is set by the 1st creator of the pipe.
With FILE_FLAG_FIRST_PIPE_INSTANCE CreateNamedPipe fails with ERROR_ACCESS_DENIED error.

Add PIPE_REJECT_REMOTE_CLIENTS to the pipe mode.
Attach API works only with local JVM processes, and the flag causes connections from remote clients are automatically rejected.