diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index cecc70c3e2d97..cce4cdb2debef 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -844,7 +844,7 @@ namespace ts {
                 (source.flags | target.flags) & SymbolFlags.Assignment) {
                 Debug.assert(source !== target);
                 if (!(target.flags & SymbolFlags.Transient)) {
-                    target = cloneSymbol(target);
+                    target = cloneSymbol(resolveSymbol(target));
                 }
                 // Javascript static-property-assignment declarations always merge, even though they are also values
                 if (source.flags & SymbolFlags.ValueModule && target.flags & SymbolFlags.ValueModule && target.constEnumOnlyModule && !source.constEnumOnlyModule) {
@@ -873,7 +873,7 @@ namespace ts {
             else if (target.flags & SymbolFlags.NamespaceModule) {
                 error(getNameOfDeclaration(source.declarations[0]), Diagnostics.Cannot_augment_module_0_with_value_exports_because_it_resolves_to_a_non_module_entity, symbolToString(target));
             }
-            else {
+            else { // error
                 const isEitherEnum = !!(target.flags & SymbolFlags.Enum || source.flags & SymbolFlags.Enum);
                 const isEitherBlockScoped = !!(target.flags & SymbolFlags.BlockScopedVariable || source.flags & SymbolFlags.BlockScopedVariable);
                 const message = isEitherEnum
@@ -942,7 +942,8 @@ namespace ts {
 
         function mergeSymbolTable(target: SymbolTable, source: SymbolTable) {
             source.forEach((sourceSymbol, id) => {
-                target.set(id, target.has(id) ? mergeSymbol(target.get(id)!, sourceSymbol) : sourceSymbol);
+                const targetSymbol = target.get(id);
+                target.set(id, targetSymbol ? mergeSymbol(targetSymbol, sourceSymbol) : sourceSymbol);
             });
         }
 
@@ -1536,8 +1537,7 @@ namespace ts {
 
                 // If we're in an external module, we can't reference value symbols created from UMD export declarations
                 if (result && isInExternalModule && (meaning & SymbolFlags.Value) === SymbolFlags.Value && !(originalLocation!.flags & NodeFlags.JSDoc)) {
-                    const decls = result.declarations;
-                    if (decls && decls.length === 1 && decls[0].kind === SyntaxKind.NamespaceExportDeclaration) {
+                    if (some(result.declarations, d => isNamespaceExportDeclaration(d) || isSourceFile(d) && !!d.symbol.globalExports)) {
                         error(errorLocation!, Diagnostics._0_refers_to_a_UMD_global_but_the_current_file_is_a_module_Consider_adding_an_import_instead, unescapeLeadingUnderscores(name)); // TODO: GH#18217
                     }
                 }
diff --git a/tests/baselines/reference/duplicateVarsAcrossFileBoundaries.symbols b/tests/baselines/reference/duplicateVarsAcrossFileBoundaries.symbols
index a1f6cdf1b3868..9cedc15a75a5c 100644
--- a/tests/baselines/reference/duplicateVarsAcrossFileBoundaries.symbols
+++ b/tests/baselines/reference/duplicateVarsAcrossFileBoundaries.symbols
@@ -34,10 +34,10 @@ var z = 0;
 
 === tests/cases/compiler/duplicateVarsAcrossFileBoundaries_4.ts ===
 module P { }
->P : Symbol(P, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 0))
+>P : Symbol(P, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 0), Decl(duplicateVarsAcrossFileBoundaries_5.ts, 2, 3))
 
 import p = P;
->p : Symbol(p, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 12), Decl(duplicateVarsAcrossFileBoundaries_5.ts, 2, 3))
+>p : Symbol(p, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 12))
 >P : Symbol(P, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 0))
 
 var q;
@@ -52,5 +52,5 @@ import q = Q;
 >Q : Symbol(Q, Decl(duplicateVarsAcrossFileBoundaries_5.ts, 0, 0))
 
 var p;
->p : Symbol(p, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 12), Decl(duplicateVarsAcrossFileBoundaries_5.ts, 2, 3))
+>p : Symbol(P, Decl(duplicateVarsAcrossFileBoundaries_4.ts, 0, 0), Decl(duplicateVarsAcrossFileBoundaries_5.ts, 2, 3))
 
diff --git a/tests/baselines/reference/exportAsNamespace_augment.errors.txt b/tests/baselines/reference/exportAsNamespace_augment.errors.txt
new file mode 100644
index 0000000000000..76cb643d62ac1
--- /dev/null
+++ b/tests/baselines/reference/exportAsNamespace_augment.errors.txt
@@ -0,0 +1,50 @@
+/a.d.ts(3,14): error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+/b.ts(6,22): error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+/b.ts(12,18): error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+/b.ts(15,1): error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+/b.ts(15,7): error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+/b.ts(15,13): error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+/b.ts(15,19): error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+
+
+==== /a.d.ts (1 errors) ====
+    export as namespace a;
+    export const x = 0;
+    export const conflict = 0;
+                 ~~~~~~~~
+!!! error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+!!! related TS6203 /b.ts:6:22: 'conflict' was also declared here.
+!!! related TS6204 /b.ts:6:22: and here.
+    
+==== /b.ts (6 errors) ====
+    import * as a2 from "./a";
+    
+    declare global {
+        namespace a {
+            export const y = 0;
+            export const conflict = 0;
+                         ~~~~~~~~
+!!! error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+!!! related TS6203 /a.d.ts:3:14: 'conflict' was also declared here.
+        }
+    }
+    
+    declare module "./a" {
+        export const z = 0;
+        export const conflict = 0;
+                     ~~~~~~~~
+!!! error TS2451: Cannot redeclare block-scoped variable 'conflict'.
+!!! related TS6203 /a.d.ts:3:14: 'conflict' was also declared here.
+    }
+    
+    a.x + a.y + a.z + a.conflict;
+    ~
+!!! error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+          ~
+!!! error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+                ~
+!!! error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+                      ~
+!!! error TS2686: 'a' refers to a UMD global, but the current file is a module. Consider adding an import instead.
+    a2.x + a2.y + a2.z + a2.conflict;
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/exportAsNamespace_augment.js b/tests/baselines/reference/exportAsNamespace_augment.js
new file mode 100644
index 0000000000000..6186d698791ea
--- /dev/null
+++ b/tests/baselines/reference/exportAsNamespace_augment.js
@@ -0,0 +1,32 @@
+//// [tests/cases/compiler/exportAsNamespace_augment.ts] ////
+
+//// [a.d.ts]
+export as namespace a;
+export const x = 0;
+export const conflict = 0;
+
+//// [b.ts]
+import * as a2 from "./a";
+
+declare global {
+    namespace a {
+        export const y = 0;
+        export const conflict = 0;
+    }
+}
+
+declare module "./a" {
+    export const z = 0;
+    export const conflict = 0;
+}
+
+a.x + a.y + a.z + a.conflict;
+a2.x + a2.y + a2.z + a2.conflict;
+
+
+//// [b.js]
+"use strict";
+exports.__esModule = true;
+var a2 = require("./a");
+a.x + a.y + a.z + a.conflict;
+a2.x + a2.y + a2.z + a2.conflict;
diff --git a/tests/baselines/reference/exportAsNamespace_augment.symbols b/tests/baselines/reference/exportAsNamespace_augment.symbols
new file mode 100644
index 0000000000000..27c45f371c720
--- /dev/null
+++ b/tests/baselines/reference/exportAsNamespace_augment.symbols
@@ -0,0 +1,66 @@
+=== /a.d.ts ===
+export as namespace a;
+>a : Symbol(a, Decl(a.d.ts, 0, 0))
+
+export const x = 0;
+>x : Symbol(x, Decl(a.d.ts, 1, 12))
+
+export const conflict = 0;
+>conflict : Symbol(conflict, Decl(a.d.ts, 2, 12))
+
+=== /b.ts ===
+import * as a2 from "./a";
+>a2 : Symbol(a2, Decl(b.ts, 0, 6))
+
+declare global {
+>global : Symbol(global, Decl(b.ts, 0, 26))
+
+    namespace a {
+>a : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+
+        export const y = 0;
+>y : Symbol(y, Decl(b.ts, 4, 20))
+
+        export const conflict = 0;
+>conflict : Symbol(conflict, Decl(b.ts, 5, 20))
+    }
+}
+
+declare module "./a" {
+>"./a" : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+
+    export const z = 0;
+>z : Symbol(z, Decl(b.ts, 10, 16))
+
+    export const conflict = 0;
+>conflict : Symbol(conflict, Decl(b.ts, 11, 16))
+}
+
+a.x + a.y + a.z + a.conflict;
+>a.x : Symbol(a2.x, Decl(a.d.ts, 1, 12))
+>a : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+>x : Symbol(a2.x, Decl(a.d.ts, 1, 12))
+>a.y : Symbol(a2.y, Decl(b.ts, 4, 20))
+>a : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+>y : Symbol(a2.y, Decl(b.ts, 4, 20))
+>a.z : Symbol(a2.z, Decl(b.ts, 10, 16))
+>a : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+>z : Symbol(a2.z, Decl(b.ts, 10, 16))
+>a.conflict : Symbol(a2.conflict, Decl(a.d.ts, 2, 12))
+>a : Symbol(a2, Decl(a.d.ts, 0, 0), Decl(b.ts, 2, 16), Decl(b.ts, 7, 1))
+>conflict : Symbol(a2.conflict, Decl(a.d.ts, 2, 12))
+
+a2.x + a2.y + a2.z + a2.conflict;
+>a2.x : Symbol(a2.x, Decl(a.d.ts, 1, 12))
+>a2 : Symbol(a2, Decl(b.ts, 0, 6))
+>x : Symbol(a2.x, Decl(a.d.ts, 1, 12))
+>a2.y : Symbol(a2.y, Decl(b.ts, 4, 20))
+>a2 : Symbol(a2, Decl(b.ts, 0, 6))
+>y : Symbol(a2.y, Decl(b.ts, 4, 20))
+>a2.z : Symbol(a2.z, Decl(b.ts, 10, 16))
+>a2 : Symbol(a2, Decl(b.ts, 0, 6))
+>z : Symbol(a2.z, Decl(b.ts, 10, 16))
+>a2.conflict : Symbol(a2.conflict, Decl(a.d.ts, 2, 12))
+>a2 : Symbol(a2, Decl(b.ts, 0, 6))
+>conflict : Symbol(a2.conflict, Decl(a.d.ts, 2, 12))
+
diff --git a/tests/baselines/reference/exportAsNamespace_augment.types b/tests/baselines/reference/exportAsNamespace_augment.types
new file mode 100644
index 0000000000000..0e186f57c6fa1
--- /dev/null
+++ b/tests/baselines/reference/exportAsNamespace_augment.types
@@ -0,0 +1,78 @@
+=== /a.d.ts ===
+export as namespace a;
+>a : typeof import("/a")
+
+export const x = 0;
+>x : 0
+>0 : 0
+
+export const conflict = 0;
+>conflict : 0
+>0 : 0
+
+=== /b.ts ===
+import * as a2 from "./a";
+>a2 : typeof a2
+
+declare global {
+>global : typeof global
+
+    namespace a {
+>a : typeof a2
+
+        export const y = 0;
+>y : 0
+>0 : 0
+
+        export const conflict = 0;
+>conflict : 0
+>0 : 0
+    }
+}
+
+declare module "./a" {
+>"./a" : typeof a2
+
+    export const z = 0;
+>z : 0
+>0 : 0
+
+    export const conflict = 0;
+>conflict : 0
+>0 : 0
+}
+
+a.x + a.y + a.z + a.conflict;
+>a.x + a.y + a.z + a.conflict : number
+>a.x + a.y + a.z : number
+>a.x + a.y : number
+>a.x : 0
+>a : typeof a2
+>x : 0
+>a.y : 0
+>a : typeof a2
+>y : 0
+>a.z : 0
+>a : typeof a2
+>z : 0
+>a.conflict : 0
+>a : typeof a2
+>conflict : 0
+
+a2.x + a2.y + a2.z + a2.conflict;
+>a2.x + a2.y + a2.z + a2.conflict : number
+>a2.x + a2.y + a2.z : number
+>a2.x + a2.y : number
+>a2.x : 0
+>a2 : typeof a2
+>x : 0
+>a2.y : 0
+>a2 : typeof a2
+>y : 0
+>a2.z : 0
+>a2 : typeof a2
+>z : 0
+>a2.conflict : 0
+>a2 : typeof a2
+>conflict : 0
+
diff --git a/tests/cases/compiler/exportAsNamespace_augment.ts b/tests/cases/compiler/exportAsNamespace_augment.ts
new file mode 100644
index 0000000000000..2c972a32acc5e
--- /dev/null
+++ b/tests/cases/compiler/exportAsNamespace_augment.ts
@@ -0,0 +1,22 @@
+// @Filename: /a.d.ts
+export as namespace a;
+export const x = 0;
+export const conflict = 0;
+
+// @Filename: /b.ts
+import * as a2 from "./a";
+
+declare global {
+    namespace a {
+        export const y = 0;
+        export const conflict = 0;
+    }
+}
+
+declare module "./a" {
+    export const z = 0;
+    export const conflict = 0;
+}
+
+a.x + a.y + a.z + a.conflict;
+a2.x + a2.y + a2.z + a2.conflict;