packetbeat/beater: deduplicate interface configs

When a config is provided by fleet, it will contain multiple definitions
of the same interface, and in future we may allow individual datastreams
to specify their own interface, so allow multiple distinct interfaces,
but ensure that no interfaces have the same configuration by removing
duplicates.

CHANGELOG.next.asciidoc

@@ -243,6 +243,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 
 *Packetbeat*
 
+- Improve efficiency of sniffers by deduplicating interface configurations. {issue}36574[36574] {pull}36576[36576]
 
 *Winlogbeat*
 

packetbeat/beater/processor.go

@@ -220,14 +220,31 @@ func setupSniffer(id string, cfg config.Config, protocols *protos.ProtocolsStruc
 		return nil, err
 	}
 
-	for i, iface := range cfg.Interfaces {
+	// Ensure interfaces are uniquely represented so we don't listen on the
+	// same interface with multiple sniffers.
+	interfaces := make([]config.InterfaceConfig, 0, len(cfg.Interfaces))
+	seen := make(map[uint64]bool)
+	for _, iface := range cfg.Interfaces {
+		// Currently we hash on all fields in the config. We can revise this in future.
+		h, err := hashstructure.Hash(iface, nil)
+		if err != nil {
+			return nil, fmt.Errorf("could not deduplicate interface configurations: %w", err)
+		}
+		if seen[h] {
+			continue
+		}
+		seen[h] = true
+		interfaces = append(interfaces, iface)
+	}
+
+	for i, iface := range interfaces {
 		if iface.BpfFilter != "" || cfg.Flows.IsEnabled() {
 			continue
 		}
-		cfg.Interfaces[i].BpfFilter = protocols.BpfFilter(iface.WithVlans, icmp.Enabled())
+		interfaces[i].BpfFilter = protocols.BpfFilter(iface.WithVlans, icmp.Enabled())
 	}
 
-	return sniffer.New(id, false, "", decoders, cfg.Interfaces)
+	return sniffer.New(id, false, "", decoders, interfaces)
 }
 
 // CheckConfig performs a dry-run creation of a Packetbeat pipeline based