Sending a second follow-up email for a potential PhD position
It's typical to apply for multiple (10+) PhD positions simultaneously, rather than sequentially. Don't wait to hear back from this place, just send your other applications now. It's fine to ask again about your application, but this time ask for the date by which you can expect to have a decision.
Here's a suggestion for how to formulate the email (you can modify it according to the specific details of your situation):

Dear Prof. X,
Last month I applied for a PhD position in your department. Do you
have any further news about my application, or an idea of when you
might make a decision? I am really excited about this opportunity and
look forward to hearing from you soon.
Best wishes,
MDG1999