diff --git a/llvm/include/llvm/Frontend/Directive/DirectiveBase.td b/llvm/include/llvm/Frontend/Directive/DirectiveBase.td
--- a/llvm/include/llvm/Frontend/Directive/DirectiveBase.td
+++ b/llvm/include/llvm/Frontend/Directive/DirectiveBase.td
@@ -131,6 +131,10 @@
   // function.
   string alternativeName = "";
 
+  // Clauses cannot appear twice in the three allowed lists below. Also, since
+  // required implies allowed, the same clause cannot appear in both the
+  // allowedClauses and requiredClauses lists.
+
   // List of allowed clauses for the directive.
   list<VersionedClause> allowedClauses = [];
 
diff --git a/llvm/test/TableGen/directive3.td b/llvm/test/TableGen/directive3.td
--- a/llvm/test/TableGen/directive3.td
+++ b/llvm/test/TableGen/directive3.td
@@ -15,16 +15,29 @@
 def TDLC_ClauseB : Clause<"clauseb"> {
 }
 
+def TDLC_ClauseC : Clause<"clausec"> {
+}
+
+def TDLC_ClauseD : Clause<"claused"> {
+}
+
 def TDL_DirA : Directive<"dira"> {
   let allowedClauses = [
     VersionedClause<TDLC_ClauseA>,
-    VersionedClause<TDLC_ClauseB>
+    VersionedClause<TDLC_ClauseB>,
+    VersionedClause<TDLC_ClauseD>
   ];
   let allowedOnceClauses = [
-    VersionedClause<TDLC_ClauseA>
+    VersionedClause<TDLC_ClauseA>,
+    VersionedClause<TDLC_ClauseC>
+  ];
+  let requiredClauses = [
+    VersionedClause<TDLC_ClauseC>,
+    VersionedClause<TDLC_ClauseD>
   ];
   let isDefault = 1;
 }
 
 // CHECK: error: Clause TDLC_ClauseA already defined on directive TDL_DirA
+// CHECK: error: Clause TDLC_ClauseD already defined on directive TDL_DirA
 // CHECK: error: One or more clauses are defined multiple times on directive TDL_DirA
diff --git a/llvm/utils/TableGen/DirectiveEmitter.cpp b/llvm/utils/TableGen/DirectiveEmitter.cpp
--- a/llvm/utils/TableGen/DirectiveEmitter.cpp
+++ b/llvm/utils/TableGen/DirectiveEmitter.cpp
@@ -127,28 +127,40 @@
   return hasError;
 }
 
+// Check for duplicate clauses in lists. Clauses cannot appear twice in the
+// three allowed list. Also, since required implies allowed, clauses cannot
+// appear in both the allowedClauses and requiredClauses lists.
 bool HasDuplicateClausesInDirectives(const std::vector<Record *> &Directives) {
+  bool hasDuplicate = false;
   for (const auto &D : Directives) {
     Directive Dir{D};
     llvm::StringSet<> Clauses;
+    // Check for duplicates in the three allowed lists.
     if (HasDuplicateClauses(Dir.getAllowedClauses(), Dir, Clauses) ||
         HasDuplicateClauses(Dir.getAllowedOnceClauses(), Dir, Clauses) ||
-        HasDuplicateClauses(Dir.getAllowedExclusiveClauses(), Dir, Clauses) ||
+        HasDuplicateClauses(Dir.getAllowedExclusiveClauses(), Dir, Clauses)) {
+      hasDuplicate = true;
+    }
+    // Check for duplicate between allowedClauses and required
+    Clauses.clear();
+    if (HasDuplicateClauses(Dir.getAllowedClauses(), Dir, Clauses) ||
         HasDuplicateClauses(Dir.getRequiredClauses(), Dir, Clauses)) {
-      PrintFatalError(
-          "One or more clauses are defined multiple times on directive " +
-          Dir.getRecordName());
-      return true;
+      hasDuplicate = true;
     }
+    if (hasDuplicate)
+      PrintFatalError("One or more clauses are defined multiple times on"
+                      " directive " +
+                      Dir.getRecordName());
   }
-  return false;
+
+  return hasDuplicate;
 }
 
 // Check consitency of records. Return true if an error has been detected.
 // Return false if the records are valid.
 bool DirectiveLanguage::CheckRecordsValidity() const {
   if (getDirectiveLanguages().size() != 1) {
-    PrintError("A single definition of DirectiveLanguage is needed.");
+    PrintFatalError("A single definition of DirectiveLanguage is needed.");
     return true;
   }