Add guidance on duplicate Dim declarations in VBA

Clarify that VBA uses procedure-level scope and declaring the same variable more than once causes compile errors. Include examples of correct and incorrect patterns.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/vba-best-practices/SKILL.md

@@ -29,6 +29,62 @@ description:  VBA (Visual Basic for Applications) coding standards for Excel, Wo
 - **Use `vbNullString` instead of `""`** for empty string assignment; use `Len(s) = 0` to check emptiness — both are faster than comparing against `""`.
 - **Use `vb` string constants** instead of `Chr()` calls: `vbTab`, `vbLf`, `vbCr`, `vbNewLine` (fastest), `vbNullChar`.
 
+### No Duplicate `Dim` Declarations Within a Procedure
+
+VBA uses **procedure-level scope** — a variable declared anywhere inside a Sub/Function is scoped to the entire procedure. Declaring the same variable name more than once (even in separate `If/Else` branches or nested loops) causes a **compile error**.
+
+**Rule: declare every variable exactly once, at the top of the procedure.**
+
+```vb
+' ❌ Compile error — same name declared twice in different branches
+Sub BadExample()
+    If condition Then
+        Dim x As Integer   ' first declaration
+        x = 1
+    Else
+        Dim x As Integer   ' DUPLICATE — compiler rejects this
+        x = 2
+    End If
+End Sub
+
+' ✅ Correct — declared once at the top, assigned freely in any branch
+Sub GoodExample()
+    Dim x As Integer       ' single declaration
+
+    If condition Then
+        x = 1
+    Else
+        x = 2
+    End If
+End Sub
+```
+
+The same rule applies to loop variables reused across multiple `For Each` loops:
+
+```vb
+' ❌ Compile error — rawMat declared inside two separate loops
+For Each rawMat In rawMaterials        ' implicit Dim inside loop
+    allRawMaterials.Add rawMat
+Next rawMat
+
+For Each rawMat In allRawMaterials     ' duplicate Dim — compile error
+    ' ...
+Next rawMat
+
+' ✅ Correct — declare once at the top, reuse freely
+Dim rawMat As Variant
+
+For Each rawMat In rawMaterials
+    allRawMaterials.Add rawMat
+Next rawMat
+
+For Each rawMat In allRawMaterials
+    ' ...
+Next rawMat
+```
+
+**Tip:** A quick way to catch duplicates is to search for `Dim <varname>` within the same procedure. With `Option Explicit` enabled, the compiler will also surface this error on the first compile attempt.
+
 ---
 
 ## Parameters & Scope
@@ -104,6 +160,7 @@ Sheet1.Range("A1:D500").Value = arData   ' one write
 |---|---|
 | `Option Explicit` in every module | Prevents undeclared-variable bugs |
 | Descriptive names + Hungarian prefixes | Readability & self-documentation |
+| Each variable declared **once**, at procedure top | Duplicate `Dim` = compile error |
 | `ByVal`/`ByRef` always explicit | No silent side effects |
 | Custom `GoTo ErrorHandler`, not `Resume Next` | Errors surface instead of hiding |
 | `With…End With` for repeated object refs | Speed + clarity |
@@ -111,4 +168,4 @@ Sheet1.Range("A1:D500").Value = arData   ' one write
 | `vbNullString` / `Len()` for strings | Faster than `""` comparison |
 | Arrays for bulk range I/O | Orders-of-magnitude speed gain |
 | No `Option Base 1` or `Option Compare Text` | Predictable defaults |
-| No `GoTo` except for error handler label | Structured, readable flow |
+| No `GoTo` except for error handler label | Structured, readable flow |