diff --git a/spec/formatting.md b/spec/formatting.md
index 6d956bdc1e..16072adedd 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -123,11 +123,32 @@ The resolution of a _text_ or _literal_ token MUST always succeed.
 To resolve the value of a _variable_,
 its _name_ is used to identify either a local variable,
 or a variable defined elsewhere.
-If a local variable and an externally defined one use the same name,
-the local variable takes precedence.
 
-It is an error for a local variable definition to
-refer to a local variable that's defined after it in the message.
+When a _variable_is used as an _operand_,
+the _name_ of this variable MUST either be:
+1. The _name_ of a _variable_ of a _declaration_ that occurs before the
+_operand_, or:
+1. An externally defined variable.
+
+In case 1, the `entire` _declaration_ MUST appear before the _operand_.
+For example, resolution of the following declaration should fail:
+
+```
+let $var = {$var}
+```
+
+because although a _variable_ with _name_ `var` occurs before
+the expression `{$var}`, the entire declaration `let $var = {$var}`
+does not occur before the expression `{$var}`.
+
+Any _name_ MUST NOT appear as the _name_ of the _variable_
+in more than one _declaration_ within the same message.
+If the _variable_ in a _declaration_ also appears
+as the _variable_ in a previous _declaration_,
+a Variable Redefinition error MUST be emitted.
+If the _variable_ in a declaration is also
+an externally defined variable,
+a Formatting error MUST be emitted.
 
 The resolution of a _variable_ MAY fail if no value is identified for its _name_.
 If this happens, an Unresolved Variable error MUST be emitted.
@@ -230,6 +251,39 @@ rather than the _expression_ in the _selector_ or _pattern_.
 
 _Pattern selection_ is not supported for _fallback values_.
 
+When there are multiple _declarations_ for the same _variable_,
+or when a _declaration_ redefines a _variable_
+that is externally defined,
+the fallback string is formatted based on the _expression_ of
+the first local declaration of that _variable_.
+
+> For example, attempting to format the following message:
+>
+> ```
+> let $var = {|cart|}
+> let $var = {|horse|}
+> {The value is {$var}.}
+> ```
+>
+> would result in this formatted string representation:
+>
+> ```
+> The value is {|cart|}.
+> ```
+>
+> Also, attempting to format the following message:
+> ```
+> let $var = {|cart|}
+> {The value is {$var}.}
+> ```
+>
+> in a context where `var` is externally defined
+> would result in the same formatted string representation:
+>
+> ```
+> The value is {|cart|}.
+> ```
+
 ## Pattern Selection
 
 When a _message_ contains a _match_ construct with one or more _expressions_,
@@ -704,6 +758,32 @@ These are divided into the following categories:
     > when * {The value is not one.}
     > ```
 
+  - **Variable Redefinition errors** occur when
+a `declaration` contains a `variable` that is already defined
+    either by a previous local variable declaration,
+    or by an external definition.
+
+    > For example, attempting to format the following message
+    > results in a Variable Redefinition error,
+    > because there is another declaration for `var`
+    > that textually precedes
+    > the declaration of `var` to be `{|horse|}`.
+    >
+    > ```
+    > let $var = {|cart|}
+    > let $var = {|horse|}
+    > {The value is {$var}.}
+    > ```
+    >
+    > Attempting to format the following message in a context
+    > where `var` is externally defined
+    > also results in a Variable Redefinition error.
+    >
+    > ```
+    > let $var = {|horse|}
+    > {The value is {$var}.}
+    > ```
+
   - **Unknown Function errors** occur when an _expression_ includes
     a reference to a function which cannot be resolved.