diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index f7ea05836701e..4f34de95a526d 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -361,11 +361,11 @@ namespace ts {
         return false;
     }
 
-    export function filterMutate<T>(array: T[], f: (x: T) => boolean): void {
+    export function filterMutate<T>(array: T[], f: (x: T, i: number, array: T[]) => boolean): void {
         let outIndex = 0;
-        for (const item of array) {
-            if (f(item)) {
-                array[outIndex] = item;
+        for (let i = 0; i < array.length; i++) {
+            if (f(array[i], i, array)) {
+                array[outIndex] = array[i];
                 outIndex++;
             }
         }
diff --git a/src/server/project.ts b/src/server/project.ts
index 524b6c4d28d3d..ae4dfb1f5c97a 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -494,7 +494,7 @@ namespace ts.server {
             this.projectStateVersion++;
         }
 
-        private extractUnresolvedImportsFromSourceFile(file: SourceFile, result: string[]) {
+        private extractUnresolvedImportsFromSourceFile(file: SourceFile, result: Push<string>) {
             const cached = this.cachedUnresolvedImportsPerFile.get(file.path);
             if (cached) {
                 // found cached result - use it and return
@@ -554,7 +554,7 @@ namespace ts.server {
                 for (const sourceFile of this.program.getSourceFiles()) {
                     this.extractUnresolvedImportsFromSourceFile(sourceFile, result);
                 }
-                this.lastCachedUnresolvedImportsList = toSortedArray(result);
+                this.lastCachedUnresolvedImportsList = toDeduplicatedSortedArray(result);
             }
             unresolvedImports = this.lastCachedUnresolvedImportsList;
 
diff --git a/src/server/utilities.ts b/src/server/utilities.ts
index fc88f11408a93..5efb20d074fae 100644
--- a/src/server/utilities.ts
+++ b/src/server/utilities.ts
@@ -262,6 +262,15 @@ namespace ts.server {
         return arr as SortedArray<T>;
     }
 
+    export function toDeduplicatedSortedArray(arr: string[]): SortedArray<string> {
+        arr.sort();
+        filterMutate(arr, isNonDuplicateInSortedArray);
+        return arr as SortedArray<string>;
+    }
+    function isNonDuplicateInSortedArray<T>(value: T, index: number, array: T[]) {
+        return index === 0 || value !== array[index - 1];
+    }
+
     export function enumerateInsertsAndDeletes<T>(newItems: SortedReadonlyArray<T>, oldItems: SortedReadonlyArray<T>, inserted: (newItem: T) => void, deleted: (oldItem: T) => void, compare?: Comparer<T>) {
         compare = compare || compareValues;
         let newIndex = 0;
diff --git a/src/services/jsTyping.ts b/src/services/jsTyping.ts
index 4de7bb3191d10..5e7b7d424f894 100644
--- a/src/services/jsTyping.ts
+++ b/src/services/jsTyping.ts
@@ -35,7 +35,7 @@ namespace ts.JsTyping {
         "crypto", "stream", "util", "assert", "tty", "domain",
         "constants", "process", "v8", "timers", "console"];
 
-    const nodeCoreModules = arrayToMap(<string[]>nodeCoreModuleList, x => x);
+    const nodeCoreModules = arrayToSet(nodeCoreModuleList);
 
     /**
      * A map of loose file names to library names that we are confident require typings