AV0125: Consider duplicating simple logic across modules to reduce coupling

_rules/0125.md

@@ -1,13 +1,39 @@
 ---
 rule_id: 0125
 rule_category: general
-title: Don't Repeat Yourself (DRY) within boundaries
+title: Don't Repeat Yourself (DRY), but only within boundaries
 severity: 1
 ---
-Avoid duplicating logic, knowledge or decisions within a component, service or [bounded context](https://martinfowler.com/bliki/BoundedContext.html). When the same logic exists in multiple places, a change in requirements requires multiple edits, and inconsistencies creep in over time. Use the [Rule of Three](https://en.wikipedia.org/wiki/Rule_of_three_(computer_programming)) as a practical guide: refactor duplication after the third occurrence.
+Avoid duplicating logic, knowledge or decisions within a component, service or [bounded context](https://martinfowler.com/bliki/BoundedContext.html). When the same logic exists in multiple places, a change in requirements requires multiple edits, and inconsistencies creep in over time. 
 
-**Exception:**  
-Duplication across architectural or team boundaries is sometimes preferable to coupling. If sharing a piece of logic between two separate modules or services would require introducing a shared dependency, consider whether duplicating the simple logic is the lesser evil. Tight coupling across boundaries is harder to manage than a small amount of redundancy.
+However, although the DRY principle (Don't Repeat Yourself) is valuable within such a boundary, blindly applying it across module or service boundaries can introduce coupling that is harder to manage than a small amount of duplication.
+
+If sharing a tiny piece of logic between two separate modules would require:
+- Adding a dependency on a shared library that both modules must coordinate upgrades for.
+- Exposing internal types or contracts across a boundary.
+- Introducing a third project or package just to hold a few lines of code.
+
+…then duplicating the logic is often the simpler choice.
+
+```csharp
+// In Module A
+private static bool IsValidEmail(string value)
+    => value.Contains('@') && value.Contains('.');
+
+// In Module B — a small duplication is fine here
+private static bool IsValidEmail(string value)
+    => value.Contains('@') && value.Contains('.');
+```
+
+This applies primarily to:
+- Small utility functions (a few lines of code).
+- Logic that is stable and unlikely to change frequently.
+- Logic that doesn't carry domain meaning that must remain consistent.
+
+For complex or domain-critical logic that must be consistent everywhere, a shared library is still the right choice. Consider the [Rule of Three](https://en.wikipedia.org/wiki/Rule_of_three_(computer_programming)) as a practical guide: refactor duplication after the third occurrence.
+
+**Note:** 
+Taking a dependency on a source-only package like [Reflectify](https://github.com/dennisdoomen/reflectify?tab=readme-ov-file#readme) or [Pathy](https://github.com/dennisdoomen/pathy?tab=readme-ov-file#readme), or putting your utility code in a source-only package also avoids coordinated updates. But duplication may still be the safer choice. 
 
 **Exception:**  
 Duplication in tests is often beneficial as it will make the tests easier to understand without the need to dig into all kinds of shared helper methods.