diff --git a/src/relative-time-element.js b/src/relative-time-element.js
index 1aa612a..772f887 100644
--- a/src/relative-time-element.js
+++ b/src/relative-time-element.js
@@ -54,5 +54,7 @@ function updateNowElements() {
 //   var time = new RelativeTimeElement()
 //   # => <relative-time></relative-time>
 //
-window.RelativeTimeElement = RelativeTimeElement
-window.customElements.define('relative-time', RelativeTimeElement)
+if (!window.customElements.get('relative-time')) {
+  window.RelativeTimeElement = RelativeTimeElement
+  window.customElements.define('relative-time', RelativeTimeElement)
+}
diff --git a/src/time-ago-element.js b/src/time-ago-element.js
index b9f7f06..068ed0b 100644
--- a/src/time-ago-element.js
+++ b/src/time-ago-element.js
@@ -14,5 +14,7 @@ export default class TimeAgoElement extends RelativeTimeElement {
   }
 }
 
-window.TimeAgoElement = TimeAgoElement
-window.customElements.define('time-ago', TimeAgoElement)
+if (!window.customElements.get('time-ago')) {
+  window.TimeAgoElement = TimeAgoElement
+  window.customElements.define('time-ago', TimeAgoElement)
+}
diff --git a/src/time-until-element.js b/src/time-until-element.js
index b860494..0949e50 100644
--- a/src/time-until-element.js
+++ b/src/time-until-element.js
@@ -14,5 +14,7 @@ export default class TimeUntilElement extends RelativeTimeElement {
   }
 }
 
-window.TimeUntilElement = TimeUntilElement
-window.customElements.define('time-until', TimeUntilElement)
+if (!window.customElements.get('time-until')) {
+  window.TimeUntilElement = TimeUntilElement
+  window.customElements.define('time-until', TimeUntilElement)
+}