can -> MUST

.chloggen/processor-duplication-per-pipeline.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: service
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Disallow duplicate references to processors within a single pipeline
+
+# One or more tracking issues or pull requests related to the change
+issues: [6540]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

docs/design.md

@@ -149,7 +149,7 @@ When the Collector loads this config the result will look like this:
 
 Note that each `batch` processor is an independent instance, although both are configured the same way, i.e. each have a `send_batch_size` of 10000.
 
-The same name of the processor can NOT be referenced multiple times in the `processors` key of a single pipeline.
+The same name of the processor MUST NOT be referenced multiple times in the `processors` key of a single pipeline.
 
 ## <a name="opentelemetry-agent"></a>Running as an Agent
 

service/config.go

@@ -130,11 +130,18 @@ func (cfg *Config) validateService() error {
 		}
 
 		// Validate pipeline processor name references.
+		procSet := make(map[component.ID]bool, len(cfg.Processors))
 		for _, ref := range pipeline.Processors {
 			// Check that the name referenced in the pipeline's processors exists in the top-level processors.
 			if cfg.Processors[ref] == nil {
 				return fmt.Errorf("pipeline %q references processor %q which does not exist", pipelineID, ref)
 			}
+			// Ensure no processors are duplicated within the pipeline
+			if _, exists := procSet[ref]; exists {
+
+				return fmt.Errorf("pipeline %q references processor %q multiple times", pipelineID, ref)
+			}
+			procSet[ref] = true
 		}
 
 		// Validate pipeline has at least one exporter.

service/config_test.go

@@ -138,6 +138,16 @@ func TestConfigValidate(t *testing.T) {
 			},
 			expected: errors.New(`pipeline "traces" references processor "nop/2" which does not exist`),
 		},
+		{
+			name: "duplicate-processor-reference",
+			cfgFn: func() *Config {
+				cfg := generateConfig()
+				pipe := cfg.Service.Pipelines[component.NewID("traces")]
+				pipe.Processors = append(pipe.Processors, pipe.Processors...)
+				return cfg
+			},
+			expected: errors.New(`pipeline "traces" references processor "nop" multiple times`),
+		},
 		{
 			name: "invalid-exporter-reference",
 			cfgFn: func() *Config {