diff --git a/clang-tools-extra/clangd/Preamble.cpp b/clang-tools-extra/clangd/Preamble.cpp
--- a/clang-tools-extra/clangd/Preamble.cpp
+++ b/clang-tools-extra/clangd/Preamble.cpp
@@ -213,6 +213,9 @@
   // Full text that's representing the directive, including the `#`.
   std::string Text;
   unsigned Offset;
+  tok::PPKeywordKind Directive;
+  // Name of the macro being defined in the case of a #define directive.
+  std::string MacroName;
 
   bool operator==(const TextualPPDirective &RHS) const {
     return std::tie(DirectiveLine, Offset, Text) ==
@@ -283,6 +286,8 @@
       return;
     TextualDirectives.emplace_back();
     TextualPPDirective &TD = TextualDirectives.back();
+    TD.Directive = tok::pp_define;
+    TD.MacroName = MacroNameTok.getIdentifierInfo()->getName().str();
 
     const auto *MI = MD->getMacroInfo();
     TD.Text =
@@ -711,6 +716,10 @@
     // reduce complexity. The former might cause problems because scanning is
     // imprecise and might pick directives from disabled regions.
     for (const auto &TD : ModifiedScan->TextualDirectives) {
+      // Introduce an #undef directive before #defines to suppress any
+      // re-definition warnings.
+      if (TD.Directive == tok::pp_define)
+        Patch << "#undef " << TD.MacroName << '\n';
       Patch << "#line " << TD.DirectiveLine << '\n';
       Patch << TD.Text << '\n';
     }
diff --git a/clang-tools-extra/clangd/unittests/PreambleTests.cpp b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
--- a/clang-tools-extra/clangd/unittests/PreambleTests.cpp
+++ b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
@@ -251,6 +251,7 @@
         #define BAR
         [[BAR]])cpp",
           R"cpp(#line 0 ".*main.cpp"
+#undef BAR
 #line 2
 #define         BAR
 )cpp",
@@ -262,6 +263,7 @@
 
         [[BAR]])cpp",
           R"cpp(#line 0 ".*main.cpp"
+#undef BAR
 #line 2
 #define         BAR
 )cpp",
@@ -273,6 +275,7 @@
                 BAR
         [[BAR]])cpp",
           R"cpp(#line 0 ".*main.cpp"
+#undef BAR
 #line 3
 #define         BAR
 )cpp",
@@ -305,8 +308,10 @@
   )cpp");
 
   llvm::StringLiteral ExpectedPatch(R"cpp(#line 0 ".*main.cpp"
+#undef BAR
 #line 2
 #define     BAR\(X, Y\) X Y
+#undef BAR
 #line 3
 #define     BAR\(X\) X
 )cpp");
@@ -624,7 +629,7 @@
 
   llvm::StringMap<std::string> AdditionalFiles;
   AdditionalFiles["foo.h"] = "#pragma once";
-  AdditionalFiles["bar.h"] = "#pragma once";
+  AdditionalFiles["barx.h"] = "#pragma once";
   llvm::StringLiteral BaselinePreamble("#define FOO 1\n[[#include \"foo.h\"]]");
   {
     // Check with removals from preamble.
@@ -637,23 +642,23 @@
   {
     // Check with additions to preamble.
     Annotations Code(BaselinePreamble);
-    Annotations NewCode(("[[#include \"bar.h\"]]\n" + BaselinePreamble).str());
+    Annotations NewCode(("[[#include \"barx.h\"]]\n" + BaselinePreamble).str());
     auto AST = createPatchedAST(Code.code(), NewCode.code(), AdditionalFiles);
-    // FIXME: This is just all sorts of wrong. We point at the FOO from baseline
-    // claiming it's redefined and not point at the new include of "bar.h".
+    // FIXME: Should preserve existing diagnostics.
+    EXPECT_THAT(mainFileDiagRanges(*AST),
+                UnorderedElementsAre(NewCode.ranges()[0]));
   }
   {
     llvm::StringLiteral BaselinePreamble = "#define [[FOO]] 1\n";
     Annotations Code(BaselinePreamble);
     // Check ranges for notes.
-    Annotations NewCode(("#define [[BARXYZ]] 1\n" + BaselinePreamble +
+    Annotations NewCode(("#define BARXYZ 1\n" + BaselinePreamble +
                          "void foo();\n#define [[FOO]] 2")
                             .str());
     auto AST = createPatchedAST(Code.code(), NewCode.code(), AdditionalFiles);
-    // FIXME: We shouldn't be warning for BARXYZ, and pointing at the FOO inside
-    // the baselinepreamble.
+    // FIXME: We should be pointing at the first definition of FOO in NewCode.
     EXPECT_THAT(mainFileDiagRanges(*AST),
-                UnorderedElementsAre(NewCode.ranges()[0], NewCode.ranges()[2]));
+                UnorderedElementsAre(NewCode.ranges()[1]));
   }
 }