diff --git a/pkg/workflow/observability_otlp.go b/pkg/workflow/observability_otlp.go
index e2ff2fd52f1..91d93f5050a 100644
--- a/pkg/workflow/observability_otlp.go
+++ b/pkg/workflow/observability_otlp.go
@@ -17,6 +17,7 @@ var otlpLog = logger.New("workflow:observability_otlp")
 
 var sentryEndpointExpressionPattern = regexp.MustCompile(`(?i)^\$\{\{\s*secrets\.` + regexp.QuoteMeta(constants.OTELSentryEndpointSecretName) + `\s*\}\}$`)
 var otlpResourceAttributeSecretRefPattern = regexp.MustCompile(`\$\{\{\s*(secrets|vars)\.`)
+var otelServiceNameKeyPattern = regexp.MustCompile(`(?m)^\s*OTEL_SERVICE_NAME:`)
 
 func normalizeOTLPHeadersForEndpoint(raw any, endpoint string) string {
 	if raw == nil {
@@ -732,7 +733,14 @@ func (c *Compiler) injectOTLPConfig(workflowData *WorkflowData) {
 	//    OTEL_EXPORTER_OTLP_ENDPOINT is set to the first endpoint for backward
 	//    compatibility (MCP gateway, legacy scripts). OTEL_SERVICE_NAME is
 	//    workflow-specific when WorkflowID is available.
-	otlpEnvLines := fmt.Sprintf("  OTEL_EXPORTER_OTLP_ENDPOINT: %s\n  OTEL_SERVICE_NAME: %s", firstEndpoint, serviceName)
+	//    If the user has already defined OTEL_SERVICE_NAME in their env block,
+	//    we respect their value and skip injection to avoid duplicate key errors.
+	otlpEnvLines := "  OTEL_EXPORTER_OTLP_ENDPOINT: " + firstEndpoint
+	if otelServiceNameKeyPattern.MatchString(workflowData.Env) {
+		otlpLog.Printf("Skipping OTEL_SERVICE_NAME injection: already defined by user")
+	} else {
+		otlpEnvLines += "\n  OTEL_SERVICE_NAME: " + serviceName
+	}
 	otlpEnvLines += "\n  OTEL_RESOURCE_ATTRIBUTES: '" + escapeYAMLSingleQuoted(otelResourceAttributes(workflowData)) + "'"
 
 	// 3. Inject per-endpoint headers env vars.
diff --git a/pkg/workflow/observability_otlp_test.go b/pkg/workflow/observability_otlp_test.go
index 3b91aa08c24..40cf59b0c67 100644
--- a/pkg/workflow/observability_otlp_test.go
+++ b/pkg/workflow/observability_otlp_test.go
@@ -585,6 +585,23 @@ func TestInjectOTLPConfig(t *testing.T) {
 		assert.Equal(t, 1, strings.Count(wd.Env, "env:"), "should have exactly one env: key")
 	})
 
+	t.Run("preserves user-defined OTEL_SERVICE_NAME and does not inject duplicate", func(t *testing.T) {
+		c := newCompiler()
+		wd := &WorkflowData{
+			ParsedFrontmatter: &FrontmatterConfig{
+				Observability: &ObservabilityConfig{
+					OTLP: &OTLPConfig{Endpoint: "https://traces.example.com"},
+				},
+			},
+			Env: "env:\n  OTEL_SERVICE_NAME: my-service",
+		}
+		c.injectOTLPConfig(wd)
+
+		assert.Contains(t, wd.Env, "OTEL_SERVICE_NAME: my-service", "user-defined service name should be preserved")
+		assert.Equal(t, 1, strings.Count(wd.Env, "OTEL_SERVICE_NAME:"), "OTEL_SERVICE_NAME should appear exactly once")
+		assert.Contains(t, wd.Env, "OTEL_EXPORTER_OTLP_ENDPOINT: https://traces.example.com", "endpoint should still be injected")
+	})
+
 	t.Run("OTEL_SERVICE_NAME includes sanitized workflow ID when available", func(t *testing.T) {
 		c := newCompiler()
 		wd := &WorkflowData{