Avoid duplicating nested error causes

docs/roadmap/remediation.md

@@ -121,7 +121,12 @@ the selected self-contained contract is not satisfied.
 
 ### 3. Avoid Duplicating An Already-Retained Error Cause [P3]
 
-**Status:** Open. **Stages:** 3–4.
+**Status:** Complete. **Stages:** 3–4.
+
+Completion: ordinary operation projection retains its mapped cause once, without
+joining the original subtree a second time. Multi-engine regression coverage
+checks exact cause/owner matching and one occurrence of an independent leaf;
+specialized projection and redaction tests remain intact.
 
 **Location:** [public error projection](../../errors.go), `mapPublicError`,
 particularly lines 26–38.