--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -944,16 +944,18 @@ nsContentUtils::InitializeEventTable() {
   sAtomEventTable = new nsDataHashtable<nsISupportsHashKey, EventNameMapping>(
       ArrayLength(eventArray));
   sStringEventTable = new nsDataHashtable<nsStringHashKey, EventNameMapping>(
       ArrayLength(eventArray));
   sUserDefinedEvents = new nsCOMArray<nsIAtom>(64);
 
   // Subtract one from the length because of the trailing null
   for (uint32_t i = 0; i < ArrayLength(eventArray) - 1; ++i) {
+    MOZ_ASSERT(!sAtomEventTable->Lookup(eventArray[i].mAtom),
+               "Double-defining event name; fix your EventNameList.h");
     sAtomEventTable->Put(eventArray[i].mAtom, eventArray[i]);
     if (ShouldAddEventToStringEventTable(eventArray[i])) {
       sStringEventTable->Put(
         Substring(nsDependentAtomString(eventArray[i].mAtom), 2),
         eventArray[i]);
     }
   }