Update AV1580 guideline

Split from #298.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Dennis Doomen

_rules/1580.md

@@ -1,13 +1,31 @@
 ---
 rule_id: 1580
 rule_category: maintainability
-title: Write code that is easy to debug
-severity: 2
+title: Consider duplicating simple logic across modules to reduce coupling
+severity: 3
 ---
-Because debugger breakpoints cannot be set inside expressions, avoid overuse of nested method calls. For example, a line like:
+The DRY principle (Don't Repeat Yourself) is valuable within a bounded context, but applying it blindly across module or service boundaries can introduce coupling that is harder to manage than a small amount of duplication.
 
-	string result = ConvertToXml(ApplyTransforms(ExecuteQuery(GetConfigurationSettings(source))));
+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.
 
-requires extra steps to inspect intermediate method return values. On the other hard, were this expression broken into intermediate variables, setting a breakpoint on one of them would be sufficient.
+…then duplicating the logic is often the simpler choice.
 
-**Note** This does not apply to chaining method calls, which is a common pattern in fluent APIs.
+```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.