Added dispatch_uid argument to all receivers to prevent duplicate

execution

doc/development/api/exporter.rst

@@ -25,7 +25,7 @@ that we'll soon create::
     from pretix.base.signals import register_data_exporter
 
 
-    @receiver(register_data_exporter)
+    @receiver(register_data_exporter, dispatch_uid="exporter_myexporter")
     def register_data_exporter(sender, **kwargs):
         from .exporter import MyExporter
         return MyExporter

doc/development/api/payment.rst

@@ -21,7 +21,7 @@ that we'll soon create::
     from pretix.base.signals import register_payment_providers
 
 
-    @receiver(register_payment_providers)
+    @receiver(register_payment_providers, dispatch_uid="payment_paypal")
     def register_payment_provider(sender, **kwargs):
         from .payment import Paypal
         return Paypal

doc/development/api/restriction.rst

@@ -108,7 +108,7 @@ In our example, the implementation could look like this::
     from .models import TimeRestriction
 
 
-    @receiver(determine_availability)
+    @receiver(determine_availability, dispatch_uid="restriction_time")
     def availability_handler(sender, **kwargs):
         # Handle the signal's input arguments
         item = kwargs['item']
@@ -273,7 +273,7 @@ Our time restriction example looks like this::
             ]
 
 
-    @receiver(restriction_formset)
+    @receiver(restriction_formset, dispatch_uid="restriction_formset_time")
     def formset_handler(sender, **kwargs):
         formset = inlineformset_factory(
             Item,

doc/development/api/ticketoutput.rst

@@ -24,7 +24,7 @@ that we'll soon create::
     from pretix.base.signals import register_ticket_outputs
 
 
-    @receiver(register_ticket_outputs)
+    @receiver(register_ticket_outputs, dispatch_uid="output_pdf")
     def register_ticket_output(sender, **kwargs):
         from .ticketoutput import PdfTicketOutput
         return PdfTicketOutput