Added dispatch_uid argument to all receivers to prevent duplicate

execution

src/pretix/base/exporter.py

@@ -153,6 +153,6 @@ class JSONExporter(BaseExporter):
         return JsonResponse(jo)
 
 
-@receiver(register_data_exporters)
+@receiver(register_data_exporters, dispatch_uid="exporter_json")
 def register_json_export(sender, **kwargs):
     return JSONExporter

src/pretix/base/payment.py

@@ -445,6 +445,6 @@ class FreeOrderProvider(BasePaymentProvider):
         ).aggregate(sum=Sum('price'))['sum'] == 0
 
 
-@receiver(register_payment_providers)
+@receiver(register_payment_providers, dispatch_uid="payment_free")
 def register_payment_provider(sender, **kwargs):
     return FreeOrderProvider