diff --git a/mlir/lib/Transforms/Utils/DialectConversion.cpp b/mlir/lib/Transforms/Utils/DialectConversion.cpp
--- a/mlir/lib/Transforms/Utils/DialectConversion.cpp
+++ b/mlir/lib/Transforms/Utils/DialectConversion.cpp
@@ -1056,6 +1056,11 @@
   // Drop all of the unresolved materialization operations created during
   // conversion.
   for (auto &mat : unresolvedMaterializations) {
+    // In some cases, such as when a materialization is removed via folding,
+    // the materialization op will already have a replacement, and we need
+    // to avoid a double-drop.
+    if (replacements.count(mat.getOp()))
+      continue;
     mat.getOp()->dropAllUses();
     mat.getOp()->erase();
   }
@@ -3395,7 +3400,8 @@
 // Full Conversion
 
 LogicalResult
-mlir::applyFullConversion(ArrayRef<Operation *> ops, const ConversionTarget &target,
+mlir::applyFullConversion(ArrayRef<Operation *> ops,
+                          const ConversionTarget &target,
                           const FrozenRewritePatternSet &patterns) {
   OperationConverter opConverter(target, patterns, OpConversionMode::Full);
   return opConverter.convertOperations(ops);
diff --git a/mlir/test/IR/test-dialect-conversion-folded-materialization.mlir b/mlir/test/IR/test-dialect-conversion-folded-materialization.mlir
new file mode 100644
--- /dev/null
+++ b/mlir/test/IR/test-dialect-conversion-folded-materialization.mlir
@@ -0,0 +1,23 @@
+// A regression test for https://github.com/llvm/llvm-project/issues/64665 in
+// which the dialect conversion framework tried to remove inserted
+// unrealized_conversion_cast ops twice.
+
+// RUN: mlir-opt %s -test-drop-dozing > %t
+// RUN: FileCheck %s < %t
+
+// CHECK-LABEL: example_fn
+// CHECK-NOT: fall_asleep
+func.func @example_fn(
+    %s1 : !test.dozing<i32>,
+    %s2 : !test.dozing<i32>) -> !test.dozing<i32> {
+  func.return %s1 : !test.dozing<i32>
+}
+
+func.func @test_convert_call() {
+  %0 = arith.constant 7 : i32
+  %1 = arith.constant 8 : i32
+  %2 = test.fall_asleep %0 : i32 -> !test.dozing<i32>
+  %3 = test.fall_asleep %1 : i32 -> !test.dozing<i32>
+  %4 = func.call @example_fn(%2, %3) : (!test.dozing<i32>, !test.dozing<i32>) -> !test.dozing<i32>
+  func.return
+}
diff --git a/mlir/test/lib/Dialect/Test/TestOps.td b/mlir/test/lib/Dialect/Test/TestOps.td
--- a/mlir/test/lib/Dialect/Test/TestOps.td
+++ b/mlir/test/lib/Dialect/Test/TestOps.td
@@ -2884,4 +2884,19 @@
   let assemblyFormat = "attr-dict";
 }
 
+//===----------------------------------------------------------------------===//
+// Test ops for dialect conversion materialization bug regression
+// https://github.com/llvm/llvm-project/issues/64665
+//===----------------------------------------------------------------------===//
+
+def TestDozing_FallAsleepOp : TEST_Op<"fall_asleep", [Pure]> {
+  let summary = "Rock the input value gently to sleep.";
+  let arguments = (ins AnyType:$input);
+  let results = (outs TestDozing:$output);
+  let assemblyFormat = "$input attr-dict `:` type($input) `->` type($output)";
+  let builders = [
+    OpBuilder<(ins "::mlir::Value":$input)>
+  ];
+}
+
 #endif // TEST_OPS
diff --git a/mlir/test/lib/Dialect/Test/TestPatterns.cpp b/mlir/test/lib/Dialect/Test/TestPatterns.cpp
--- a/mlir/test/lib/Dialect/Test/TestPatterns.cpp
+++ b/mlir/test/lib/Dialect/Test/TestPatterns.cpp
@@ -1696,6 +1696,76 @@
 };
 } // namespace
 
+//===----------------------------------------------------------------------===//
+// Test Dialect Conversion Materialization Folding
+//===----------------------------------------------------------------------===//
+
+namespace {
+
+class DropDozingTypeConverter : public TypeConverter {
+public:
+  DropDozingTypeConverter() {
+    addConversion([](Type type) { return type; });
+    addConversion(
+        [](TestDozingType type) -> Type { return type.getValueType(); });
+  }
+};
+
+struct ConvertFallAsleep : public OpConversionPattern<FallAsleepOp> {
+  ConvertFallAsleep(MLIRContext *context)
+      : OpConversionPattern<FallAsleepOp>(context) {}
+
+  using OpConversionPattern::OpConversionPattern;
+
+  LogicalResult
+  matchAndRewrite(FallAsleepOp op, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    rewriter.replaceAllUsesWith(op.getResult(), adaptor.getInput());
+    rewriter.eraseOp(op);
+    return success();
+  }
+};
+
+struct TestDialectConversionMaterializationFolding
+    : public PassWrapper<TestDialectConversionMaterializationFolding,
+                         OperationPass<>> {
+  MLIR_DEFINE_EXPLICIT_INTERNAL_INLINE_TYPE_ID(
+      TestDialectConversionMaterializationFolding)
+
+  StringRef getArgument() const final { return "test-drop-dozing"; }
+  StringRef getDescription() const final {
+    return "Test dropping dozing types";
+  }
+  void runOnOperation() override {
+    MLIRContext *context = &getContext();
+    Operation *op = getOperation();
+    RewritePatternSet patterns(context);
+    DropDozingTypeConverter typeConverter;
+    ConversionTarget target(*context);
+
+    target.addIllegalOp<FallAsleepOp>();
+    target.addDynamicallyLegalOp<func::FuncOp>([&](func::FuncOp op) {
+      return typeConverter.isSignatureLegal(op.getFunctionType()) &&
+             typeConverter.isLegal(&op.getBody());
+    });
+    target.addDynamicallyLegalOp<func::CallOp>(
+        [&](func::CallOp op) { return typeConverter.isLegal(op); });
+    target.addDynamicallyLegalOp<func::ReturnOp>(
+        [&](func::ReturnOp op) { return typeConverter.isLegal(op); });
+
+    patterns.add<ConvertFallAsleep>(typeConverter, context);
+    populateFunctionOpInterfaceTypeConversionPattern<func::FuncOp>(
+        patterns, typeConverter);
+    populateReturnOpTypeConversionPattern(patterns, typeConverter);
+    populateCallOpTypeConversionPattern(patterns, typeConverter);
+
+    if (failed(applyPartialConversion(op, target, std::move(patterns)))) {
+      signalPassFailure();
+    }
+  }
+};
+} // namespace
+
 //===----------------------------------------------------------------------===//
 // PassRegistration
 //===----------------------------------------------------------------------===//
@@ -1725,6 +1795,7 @@
 
   PassRegistration<TestMergeBlocksPatternDriver>();
   PassRegistration<TestSelectiveReplacementPatternDriver>();
+  PassRegistration<TestDialectConversionMaterializationFolding>();
 }
 } // namespace test
 } // namespace mlir
diff --git a/mlir/test/lib/Dialect/Test/TestTypeDefs.td b/mlir/test/lib/Dialect/Test/TestTypeDefs.td
--- a/mlir/test/lib/Dialect/Test/TestTypeDefs.td
+++ b/mlir/test/lib/Dialect/Test/TestTypeDefs.td
@@ -373,4 +373,12 @@
   let mnemonic = "i32";
 }
 
+def TestDozing : Test_Type<"TestDozing"> {
+  let mnemonic = "dozing";
+  let summary = "An MLIR type, but very tired.";
+  let parameters = (ins "Type":$valueType);
+  let assemblyFormat = "`<` $valueType `>`";
+}
+
+
 #endif // TEST_TYPEDEFS