diff --git a/README.md b/README.md
index ee8013c..80070b3 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,7 @@ JSON ESLint config example:
 - [No DOM Traversal in Connectedcallback](./docs/rules/no-dom-traversal-in-connectedcallback.md)
 - [No Exports with Element](./docs/rules/no-exports-with-element.md)
 - [No Method Prefixed with on](./docs/rules/no-method-prefixed-with-on.md)
+- [No Unchecked Define](./docs/rules/no-unchecked-define.md)
 - [One Element Per File](./docs/rules/one-element-per-file.md)
 - [Tag Name Matches Class](./docs/rules/tag-name-matches-class.md)
 - [Valid Tag Name](./docs/rules/valid-tag-name.md)
diff --git a/docs/rules/no-unchecked-define.md b/docs/rules/no-unchecked-define.md
new file mode 100644
index 0000000..71ff71a
--- /dev/null
+++ b/docs/rules/no-unchecked-define.md
@@ -0,0 +1,31 @@
+# No Unchecked Define
+
+Registering a custom element under the same tag as another defined custom element will cause a runtime exception in browsers.
+
+Some JavaScript might be inadvertently loaded twice on the same page in a web application, causing run time errors when the same element is registered twice.
+
+## Rule Details
+
+This rule ensures that all custom element definition calls are wrapped in guards if that element is already defined.
+
+👎 Examples of **incorrect** code for this rule:
+
+```js
+window.customElements.define('foo-bar', class extends HTMLElement {})
+```
+
+👍 Examples of **correct** code for this rule:
+
+```js
+if (!window.customElements.get('foo-bar')) {
+  window.customElements.define('foo-bar', class extends HTMLElement {})
+}
+```
+
+## When Not To Use It
+
+If you are comfortable with the trade-offs of not checking if a custom element exists before defining it.
+
+## Version
+
+This rule was introduced in v0.0.1
diff --git a/lib/rules.js b/lib/rules.js
index 076fa37..d8b7043 100644
--- a/lib/rules.js
+++ b/lib/rules.js
@@ -8,6 +8,7 @@ module.exports = {
   'no-dom-traversal-in-connectedcallback': require('./rules/no-dom-traversal-in-connectedcallback'),
   'no-exports-with-element': require('./rules/no-exports-with-element'),
   'no-method-prefixed-with-on': require('./rules/no-method-prefixed-with-on'),
+  'no-unchecked-define': require('./rules/no-unchecked-define'),
   'one-element-per-file': require('./rules/one-element-per-file'),
   'tag-name-matches-class': require('./rules/tag-name-matches-class'),
   'valid-tag-name': require('./rules/valid-tag-name')
diff --git a/lib/rules/no-unchecked-define.js b/lib/rules/no-unchecked-define.js
new file mode 100644
index 0000000..807f189
--- /dev/null
+++ b/lib/rules/no-unchecked-define.js
@@ -0,0 +1,41 @@
+const s = require('../custom-selectors')
+
+let definedCustomElements = new Map()
+
+module.exports = {
+  meta: {
+    type: 'layout',
+    docs: {description: '', url: require('../url')(module)}
+  },
+  schema: [],
+  create(context) {
+    definedCustomElements = new Map()
+    return {
+      [`IfStatement:matches([test.type=UnaryExpression],[test.type=BinaryExpression]) ${s.customElements.get}`](node) {
+        if (node.parent.type === 'UnaryExpression') {
+          let unaryCounter = 0
+          let parent = node.parent
+          while (parent.type === 'UnaryExpression') {
+            unaryCounter++
+            parent = parent.parent
+          }
+          if (unaryCounter % 2 !== 0) {
+            definedCustomElements.set(node.arguments[0].value, node)
+          }
+        } else {
+          definedCustomElements.set(node.arguments[0].value, node)
+        }
+      },
+      [s.customElements.define](node) {
+        if (definedCustomElements.has(node.arguments[0].value)) {
+          definedCustomElements.delete(node.arguments[0].value)
+        } else {
+          context.report(
+            node,
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined'
+          )
+        }
+      }
+    }
+  }
+}
diff --git a/test/no-unchecked-define.js b/test/no-unchecked-define.js
new file mode 100644
index 0000000..4ca091f
--- /dev/null
+++ b/test/no-unchecked-define.js
@@ -0,0 +1,83 @@
+const rule = require('../lib/rules/no-unchecked-define')
+const RuleTester = require('eslint').RuleTester
+
+const ruleTester = new RuleTester({env: {es2020: true}})
+ruleTester.run('no-unchecked-define', rule, {
+  valid: [
+    {
+      code:
+        'if (!window.customElements.get("foo-bar")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } '
+    },
+    {
+      code:
+        'if (!customElements.get("foo-bar")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } '
+    },
+    {
+      code:
+        'if (customElements.get("foo-bar") == null) { window.customElements.define("foo-bar", class extends HTMLElement {}) } '
+    },
+    {
+      code:
+        'if (customElements.get("foo-bar") == undefined) { window.customElements.define("foo-bar", class extends HTMLElement {}) } '
+    },
+    {
+      code:
+        'if (!!!customElements.get("foo-bar")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } '
+    }
+  ],
+  invalid: [
+    {
+      code: 'window.customElements.define("foo-bar", class extends HTMLElement {})',
+      errors: [
+        {
+          message:
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined',
+          type: 'CallExpression'
+        }
+      ]
+    },
+    {
+      code:
+        'if (customElements.get("foo-bar")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } ',
+      errors: [
+        {
+          message:
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined',
+          type: 'CallExpression'
+        }
+      ]
+    },
+    {
+      code:
+        'if (!customElements.get("bar-foo")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } ',
+      errors: [
+        {
+          message:
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined',
+          type: 'CallExpression'
+        }
+      ]
+    },
+    {
+      code: 'customElements.define("foo-bar", class extends HTMLElement {})',
+      errors: [
+        {
+          message:
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined',
+          type: 'CallExpression'
+        }
+      ]
+    },
+    {
+      code:
+        'if (!!customElements.get("foo-bar")) { window.customElements.define("foo-bar", class extends HTMLElement {}) } ',
+      errors: [
+        {
+          message:
+            'Make sure to wrap customElements.define calls in checks to see if the element has already been defined',
+          type: 'CallExpression'
+        }
+      ]
+    }
+  ]
+})