diff --git a/src/lint/rules/variable-use-def.ts b/src/lint/rules/variable-use-def.ts
index b2addcc9..3bef1a6f 100644
--- a/src/lint/rules/variable-use-def.ts
+++ b/src/lint/rules/variable-use-def.ts
@@ -9,6 +9,26 @@ import type { Reporter } from '../algorithm-error-reporter-type';
 import { Seq, walk as walkExpr } from '../../expr-parser';
 import { offsetToLineAndColumn } from '../../utils';
 
+/*
+Ecmaspeak scope rules are a bit weird.
+Variables can be declared across branches and used subsequently, as in
+
+1. If condition, then
+  1. Let _var_ be true.
+1. Else,
+  1. Let _var_ be false.
+1. Use _var_.
+
+So it mostly behaves like `var`-declared variables in JS.
+
+(Exception: loop variables can't be used outside the loop.)
+
+But for readability reasons, we don't want to allow redeclaration or shadowing.
+
+The `Scope` class tracks names as if they are `var`-scoped, and the `strictScopes` property is
+used to track names as if they are `let`-scoped, so we can warn on redeclaration.
+*/
+
 type HasLocation = { location: { start: { line: number; column: number } } };
 type VarKind =
   | 'parameter'
@@ -19,14 +39,17 @@ type VarKind =
   | 'attribute declaration';
 class Scope {
   declare vars: Map<string, { kind: VarKind; used: boolean; node: HasLocation | null }>;
+  declare strictScopes: Set<string>[];
   declare report: Reporter;
   constructor(report: Reporter) {
     this.vars = new Map();
+    this.strictScopes = [new Set()];
+    this.report = report;
+
     // TODO remove this when regex state objects become less dumb
     for (const name of ['captures', 'input', 'startIndex', 'endIndex']) {
-      this.declare(name, null);
+      this.declare(name, null, undefined, true);
     }
-    this.report = report;
   }
 
   declared(name: string): boolean {
@@ -38,11 +61,32 @@ class Scope {
     return this.vars.get(name)!.used;
   }
 
-  declare(name: string, nameNode: HasLocation | null, kind: VarKind = 'variable'): void {
+  declare(
+    name: string,
+    nameNode: HasLocation | null,
+    kind: VarKind = 'variable',
+    mayBeShadowed: boolean = false
+  ): void {
     if (this.declared(name)) {
-      return;
+      if (!mayBeShadowed) {
+        for (const scope of this.strictScopes) {
+          if (scope.has(name)) {
+            this.report({
+              ruleId: 're-declaration',
+              message: `${JSON.stringify(name)} is already declared`,
+              line: nameNode!.location.start.line,
+              column: nameNode!.location.start.column,
+            });
+            return;
+          }
+        }
+      }
+    } else {
+      this.vars.set(name, { kind, used: false, node: nameNode });
+    }
+    if (!mayBeShadowed) {
+      this.strictScopes[this.strictScopes.length - 1].add(name);
     }
-    this.vars.set(name, { kind, used: false, node: nameNode });
   }
 
   undeclare(name: string) {
@@ -100,7 +144,7 @@ export function checkVariableUsage(
     ) {
       // `__` is for <del>_x_</del><ins>_y_</ins>, which has textContent `_x__y_`
       for (const name of preceding.textContent.matchAll(/(?<=\b|_)_([a-zA-Z0-9]+)_(?=\b|_)/g)) {
-        scope.declare(name[1], null);
+        scope.declare(name[1], null, undefined, true);
       }
     }
     preceding = previousOrParent(preceding, parentClause);
@@ -145,6 +189,7 @@ function walkAlgorithm(
     }
     return;
   }
+  scope.strictScopes.push(new Set());
 
   stepLoop: for (const step of steps.contents) {
     const loopVars: Set<UnderscoreNode> = new Set();
@@ -175,7 +220,12 @@ function walkAlgorithm(
             column,
           });
         } else {
-          scope.declare(name, { location: { start: { line, column } } }, 'attribute declaration');
+          scope.declare(
+            name,
+            { location: { start: { line, column } } },
+            'attribute declaration',
+            true
+          );
         }
       }
     }
@@ -327,8 +377,12 @@ function walkAlgorithm(
               (isSuchThat && cur.name === 'text' && !/(?: of | in )/.test(cur.contents)) ||
               (isBe && cur.name === 'text' && /\blet (?:each of )?$/i.test(cur.contents))
             ) {
+              const conditional =
+                expr.items[0].name === 'text' &&
+                /^(If|Else|Otherwise)\b/.test(expr.items[0].contents);
+
               for (const v of varsDeclaredHere) {
-                scope.declare(v.contents, v);
+                scope.declare(v.contents, v, 'variable', conditional);
                 declaredThisLine.add(v);
               }
             }
@@ -353,6 +407,7 @@ function walkAlgorithm(
       scope.undeclare(decl.contents);
     }
   }
+  scope.strictScopes.pop();
 }
 
 function isVariable(node: UnderscoreNode | { name: string } | null): node is UnderscoreNode {
diff --git a/test/expr-parser.js b/test/expr-parser.js
index 56cd57b0..c9825d91 100644
--- a/test/expr-parser.js
+++ b/test/expr-parser.js
@@ -99,9 +99,9 @@ describe('expression parsing', () => {
       await assertLintFree(`
         <emu-alg>
         1. Let _x_ be a new Record { [[Foo]]: 0, <!-- comment --> [[Bar]]: 1 }.
-        1. Let _x_ be a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1</ins> }.
-        1. Let _x_ be a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1, [[Baz]]: 2</ins> }.
-        1. Let _x_ be a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1,</ins> [[Baz]]: 2 }.
+        1. Set _x_ to a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1</ins> }.
+        1. Set _x_ to a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1, [[Baz]]: 2</ins> }.
+        1. Set _x_ to a new Record { [[Foo]]: 0, <ins>[[Bar]]: 1,</ins> [[Baz]]: 2 }.
         1. Use _x_.
         </emu-alg>
       `);
diff --git a/test/lint-variable-use-def.js b/test/lint-variable-use-def.js
index 18007e90..72b9703b 100644
--- a/test/lint-variable-use-def.js
+++ b/test/lint-variable-use-def.js
@@ -497,3 +497,95 @@ describe('variables are declared and used appropriately', () => {
     });
   });
 });
+
+describe('variables cannot be redeclared', () => {
+  it('redeclaration at the same level is an error', async () => {
+    await assertLint(
+      positioned`
+        <emu-alg>
+          1. Let _x_ be 0.
+          1. Let ${M}_x_ be 1.
+          1. Return _x_.
+        </emu-alg>
+      </emu-clause>
+      `,
+      {
+        ruleId: 're-declaration',
+        nodeType: 'emu-alg',
+        message: '"x" is already declared',
+      }
+    );
+  });
+
+  it('redeclaration at an inner level is an error', async () => {
+    await assertLint(
+      positioned`
+        <emu-alg>
+          1. Let _x_ be 0.
+          1. Repeat,
+            1. Let ${M}_x_ be 1.
+          1. Return _x_.
+        </emu-alg>
+      </emu-clause>
+      `,
+      {
+        ruleId: 're-declaration',
+        nodeType: 'emu-alg',
+        message: '"x" is already declared',
+      }
+    );
+  });
+
+  it('multi-line if-else does not count as redeclaration', async () => {
+    await assertLintFree(
+      `
+      <emu-alg>
+        1. If condition, then
+          1. Let _result_ be 0.
+        1. Else,
+          1. Let _result_ be 1.
+        1. Return _result_.
+      </emu-alg>
+      `
+    );
+  });
+
+  it('single-line if-else does not count as redeclaration', async () => {
+    await assertLintFree(
+      `
+      <emu-alg>
+        1. If condition, let _result_ be 0.
+        1. Else, let _result_ be 1.
+        1. Return _result_.
+      </emu-alg>
+      `
+    );
+  });
+
+  it('[declared] annotation can be redeclared', async () => {
+    await assertLintFree(
+      `
+      <emu-alg>
+        1. [declared="var"] NOTE: Something about _var_.
+        1. Let _var_ be 0.
+        1. Return _var_.
+      </emu-alg>
+      `
+    );
+  });
+
+  it('variables mentioned in the premable can be redeclared', async () => {
+    await assertLintFree(
+      `
+      <emu-clause id="example">
+        <h1>Example</h1>
+        <p>In the following algorithm, _var_ is a variable.</p>
+        <emu-alg>
+          1. Let _var_ be 0.
+          1. Return _var_.
+        </emu-alg>
+      </emu-clause>
+      `
+    );
+  });
+});
diff --git a/test/typecheck.js b/test/typecheck.js
index 0d9ed66f..c2d2a7fe 100644
--- a/test/typecheck.js
+++ b/test/typecheck.js
@@ -131,12 +131,12 @@ describe('typechecking completions', () => {
           </dl>
           <emu-alg>
             1. Let _a_ be Completion(ExampleAlg()).
-            1. Let _a_ be Completion(<emu-meta suppress-effects="user-code">ExampleAlg()</emu-meta>).
+            1. Set _a_ to Completion(<emu-meta suppress-effects="user-code">ExampleAlg()</emu-meta>).
             1. Set _a_ to ! ExampleAlg().
             1. Return ? ExampleAlg().
             1. Let _foo_ be 0.
-            1. Let _a_ be Completion(ExampleSDO of _foo_).
-            1. Let _a_ be Completion(ExampleSDO of _foo_ with argument 0).
+            1. Set _a_ to Completion(ExampleSDO of _foo_).
+            1. Set _a_ to Completion(ExampleSDO of _foo_ with argument 0).
             1. If ? ExampleSDO of _foo_ is *true*, then
               1. Something.
           </emu-alg>