Add detection for multiple loads of the package

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Unreleased
 
+### Features
+
+-   TypeDoc will now detect and warn if multiple instances of the package are loaded. This usually means that a plugin has its own version of TypeDoc installed, which will lead to things breaking in unexpected ways.
+    It will only work if both loaded TypeDocs are v0.22.9 or later.
+
 ### Bug Fixes
 
 -   Corrected HTML generation for projects using Google Analytics, #1786.

src/lib/application.ts

@@ -33,6 +33,7 @@ import {
     getWatchEntryPoints,
 } from "./utils/entry-point";
 import { nicePath } from "./utils/paths";
+import { hasBeenLoadedMultipleTimes } from "./utils/general";
 
 // eslint-disable-next-line @typescript-eslint/no-var-requires
 const packageInfo = require("../../package.json") as {
@@ -149,6 +150,12 @@ export class Application extends ChildableComponent<
             }
         }
         this.options.read(this.logger);
+
+        if (hasBeenLoadedMultipleTimes()) {
+            this.logger.warn(
+                `TypeDoc has been loaded multiple times. This is commonly caused by plugins which have their own installation of TypeDoc. This will likely break things.`
+            );
+        }
     }
 
     /**

src/lib/utils/general.ts

@@ -32,3 +32,18 @@ export type IfInternal<T, F> = InternalOnly extends true ? T : F;
  * See {@link IfInternal} for the rationale.
  */
 export type NeverIfInternal<T> = IfInternal<never, T>;
+
+/**
+ * This is a hack to make it possible to detect and warn about installation setups
+ * which result in TypeDoc being installed multiple times. If TypeDoc has been loaded
+ * multiple times, then parts of it will not work as expected.
+ */
+const loadSymbol = Symbol.for("typedoc_loads");
+const getLoads = () => globalThis[loadSymbol as never] || 0;
+
+// @ts-expect-error
+globalThis[loadSymbol] = getLoads() + 1;
+
+export function hasBeenLoadedMultipleTimes() {
+    return getLoads() !== 1;
+}