diff --git a/lib/rules/no-duplicate-case.js b/lib/rules/no-duplicate-case.js
index 93c8548f91a..c8a0fa9da3c 100644
--- a/lib/rules/no-duplicate-case.js
+++ b/lib/rules/no-duplicate-case.js
@@ -33,17 +33,19 @@ module.exports = {
 
         return {
             SwitchStatement(node) {
-                const mapping = {};
+                const previousKeys = new Set();
 
-                node.cases.forEach(switchCase => {
-                    const key = sourceCode.getText(switchCase.test);
+                for (const switchCase of node.cases) {
+                    if (switchCase.test) {
+                        const key = sourceCode.getText(switchCase.test);
 
-                    if (mapping[key]) {
-                        context.report({ node: switchCase, messageId: "unexpected" });
-                    } else {
-                        mapping[key] = switchCase;
+                        if (previousKeys.has(key)) {
+                            context.report({ node: switchCase, messageId: "unexpected" });
+                        } else {
+                            previousKeys.add(key);
+                        }
                     }
-                });
+                }
             }
         };
     }
diff --git a/tests/lib/rules/no-duplicate-case.js b/tests/lib/rules/no-duplicate-case.js
index 21f6267a047..a6e8e0ff34c 100644
--- a/tests/lib/rules/no-duplicate-case.js
+++ b/tests/lib/rules/no-duplicate-case.js
@@ -29,71 +29,105 @@ ruleTester.run("no-duplicate-case", rule, {
         "var a = 1, f = function(s) { return { p1: s } }; switch (a) {case f(a + 1).p1: break; case f(a + 2).p1: break; default: break;}",
         "var a = 1, f = function(s) { return { p1: s } }; switch (a) {case f(a == 1 ? 2 : 3).p1: break; case f(a === 1 ? 2 : 3).p1: break; default: break;}",
         "var a = 1, f1 = function() { return { p1: 1 } }, f2 = function() { return { p1: 2 } }; switch (a) {case f1().p1: break; case f2().p1: break; default: break;}",
-        "var a = [1,2]; switch(a.toString()){case ([1,2]).toString():break; case ([1]).toString():break; default:break;}"
+        "var a = [1,2]; switch(a.toString()){case ([1,2]).toString():break; case ([1]).toString():break; default:break;}",
+        "switch(a) { case a: break; } switch(a) { case a: break; }",
+        "switch(a) { case toString: break; }"
     ],
     invalid: [
         {
             code: "var a = 1; switch (a) {case 1: break; case 1: break; case 2: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 39
             }]
         },
         {
             code: "var a = '1'; switch (a) {case '1': break; case '1': break; case '2': break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 43
             }]
         },
         {
             code: "var a = 1, one = 1; switch (a) {case one: break; case one: break; case 2: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 50
             }]
         },
         {
             code: "var a = 1, p = {p: {p1: 1, p2: 1}}; switch (a) {case p.p.p1: break; case p.p.p1: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 69
             }]
         },
         {
             code: "var a = 1, f = function(b) { return b ? { p1: 1 } : { p1: 2 }; }; switch (a) {case f(true).p1: break; case f(true).p1: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 103
             }]
         },
         {
             code: "var a = 1, f = function(s) { return { p1: s } }; switch (a) {case f(a + 1).p1: break; case f(a + 1).p1: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 87
             }]
         },
         {
             code: "var a = 1, f = function(s) { return { p1: s } }; switch (a) {case f(a === 1 ? 2 : 3).p1: break; case f(a === 1 ? 2 : 3).p1: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 97
             }]
         },
         {
             code: "var a = 1, f1 = function() { return { p1: 1 } }; switch (a) {case f1().p1: break; case f1().p1: break; default: break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 83
             }]
         },
         {
             code: "var a = [1, 2]; switch(a.toString()){case ([1, 2]).toString():break; case ([1, 2]).toString():break; default:break;}",
             errors: [{
                 messageId: "unexpected",
-                type: "SwitchCase"
+                type: "SwitchCase",
+                column: 70
             }]
+        },
+        {
+            code: "switch (a) { case a: case a: }",
+            errors: [{
+                messageId: "unexpected",
+                type: "SwitchCase",
+                column: 22
+            }]
+        },
+        {
+            code: "switch (a) { case a: break; case b: break; case a: break; case c: break; case a: break; }",
+            errors: [
+                {
+                    messageId: "unexpected",
+                    type: "SwitchCase",
+                    column: 44
+                },
+                {
+                    messageId: "unexpected",
+                    type: "SwitchCase",
+                    column: 74
+                }
+            ]
         }
     ]
 });