diff --git a/llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h b/llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h
--- a/llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h
@@ -220,6 +220,12 @@
           NumOperands(NumOperands) {
     }
 
+    /// Check if this mapping has the same \p OperandsMapping as another \p
+    /// Mapping.
+    bool isCompatibleMapping(const InstructionMapping &Mapping) const {
+      return this->OperandsMapping == Mapping.OperandsMapping;
+    }
+
     /// Default constructor.
     /// Use this constructor to express that the mapping is invalid.
     InstructionMapping() = default;
diff --git a/llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp b/llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp
--- a/llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/RegisterBankInfo.cpp
@@ -421,8 +421,19 @@
 
   // Then the alternative mapping, if any.
   InstructionMappings AltMappings = getInstrAlternativeMappings(MI);
-  for (const InstructionMapping *AltMapping : AltMappings)
-    PossibleMappings.push_back(AltMapping);
+  for (const InstructionMapping *AltMapping : AltMappings) {
+    // Check that we don't already have such a mapping.
+    bool AlreadyExists = false;
+    for (const InstructionMapping *Mapping : PossibleMappings) {
+      if (AltMapping->isCompatibleMapping(*Mapping)) {
+        AlreadyExists = true;
+        break;
+      }
+    }
+    if (!AlreadyExists) {
+      PossibleMappings.push_back(AltMapping);
+    }
+  }
 #ifndef NDEBUG
   for (const InstructionMapping *Mapping : PossibleMappings)
     assert(Mapping->verify(MI) && "Mapping is invalid");