Remove duplicate JSDoc comments

src/compiler/core.ts

@@ -129,6 +129,22 @@ namespace ts {
         return map;
     }
 
+    /**
+     * Creates a new array with `element` interspersed in between each element of `input`
+     * if there is more than 1 value in `input`. Otherwise, returns the existing array.
+     */
+    export function intersperse<T>(input: T[], element: T): T[] {
+        if (input.length <= 1) {
+            return input;
+        }
+        const result: T[] = [];
+        for (let i = 0, n = input.length; i < n; i++) {
+            if (i) result.push(element);
+            result.push(input[i]);
+        }
+        return result;
+    }
+
     /**
      * Iterates through `array` by index and performs the callback on each element of array until the callback
      * returns a falsey value, then returns false.

src/services/jsDoc.ts

@@ -89,17 +89,14 @@ namespace ts.JsDoc {
         // Eg. const a: Array<string> | Array<number>; a.length
         // The property length will have two declarations of property length coming
         // from Array<T> - Array<string> and Array<number>
-        const documentationComment: SymbolDisplayPart[] = [];
+        const documentationComment: string[] = [];
         forEachUnique(declarations, declaration => {
             for (const { comment } of getCommentHavingNodes(declaration)) {
                 if (comment === undefined) continue;
-                if (documentationComment.length) {
-                    documentationComment.push(lineBreakPart());
-                }
-                documentationComment.push(textPart(comment));
+                pushIfUnique(documentationComment, comment);
             }
         });
-        return documentationComment;
+        return intersperse(map(documentationComment, textPart), lineBreakPart());
     }
 
     function getCommentHavingNodes(declaration: Declaration): readonly (JSDoc | JSDocTag)[] {

tests/baselines/reference/quickInfoOnUnionPropertiesWithIdenticalJSDocComments01.baseline

@@ -0,0 +1,60 @@
+[
+  {
+    "marker": {
+      "fileName": "/tests/cases/fourslash/quickInfoOnUnionPropertiesWithIdenticalJSDocComments01.ts",
+      "position": 746
+    },
+    "quickInfo": {
+      "kind": "property",
+      "kindModifiers": "optional",
+      "textSpan": {
+        "start": 746,
+        "length": 8
+      },
+      "displayParts": [
+        {
+          "text": "(",
+          "kind": "punctuation"
+        },
+        {
+          "text": "property",
+          "kind": "text"
+        },
+        {
+          "text": ")",
+          "kind": "punctuation"
+        },
+        {
+          "text": " ",
+          "kind": "space"
+        },
+        {
+          "text": "language",
+          "kind": "propertyName"
+        },
+        {
+          "text": "?",
+          "kind": "punctuation"
+        },
+        {
+          "text": ":",
+          "kind": "punctuation"
+        },
+        {
+          "text": " ",
+          "kind": "space"
+        },
+        {
+          "text": "string",
+          "kind": "keyword"
+        }
+      ],
+      "documentation": [
+        {
+          "text": "A language id, like `typescript`.",
+          "kind": "text"
+        }
+      ]
+    }
+  }
+]

tests/cases/fourslash/quickInfoOnUnionPropertiesWithIdenticalJSDocComments01.ts

@@ -0,0 +1,29 @@
+/// <reference path="fourslash.ts" />
+
+////export type DocumentFilter = {
+////    /** A language id, like `typescript`. */
+////    language: string;
+////    /** A Uri [scheme](#Uri.scheme), like `file` or `untitled`. */
+////    scheme?: string;
+////    /** A glob pattern, like `*.{ts,js}`. */
+////    pattern?: string;
+////} | {
+////    /** A language id, like `typescript`. */
+////    language?: string;
+////    /** A Uri [scheme](#Uri.scheme), like `file` or `untitled`. */
+////    scheme: string;
+////    /** A glob pattern, like `*.{ts,js}`. */
+////    pattern?: string;
+////} | {
+////    /** A language id, like `typescript`. */
+////    language?: string;
+////    /** A Uri [scheme](#Uri.scheme), like `file` or `untitled`. */
+////    scheme?: string;
+////    /** A glob pattern, like `*.{ts,js}`. */
+////    pattern: string;
+////};
+////
+////declare let x: DocumentFilter;
+////x./**/language
+
+verify.baselineQuickInfo();