Added dispatch_uid argument to all receivers to prevent duplicate

execution

src/pretix/plugins/ticketoutputpdf/signals.py

@@ -3,7 +3,7 @@ from django.dispatch import receiver
 from pretix.base.signals import register_ticket_outputs
 
 
-@receiver(register_ticket_outputs)
+@receiver(register_ticket_outputs, dispatch_uid="output_pdf")
 def register_ticket_outputs(sender, **kwargs):
     from .ticketoutput import PdfTicketOutput
     return PdfTicketOutput