fix(handlers): take the same handler registered twice quietly

README.md

@@ -175,6 +175,9 @@ Gotchas:
   it is not namespaced for you. A collision does not throw: the second
   registration is silently dropped with a console warning, so pick something
   specific to your app (`myapp-image`, not `image`).
+- Registering the same handler again, with the same `id` and `tagname`, is
+  quietly ignored. That is what happens when several copies of the package
+  on a page register the defaults, so there is nothing to guard against.
 - Registering after the viewer has already read the handler list is not an
   error either — the handler just never appears in the "Open with …" menu.
   See [step 3](#3-load-your-registration-before-the-viewer) below for why

__tests__/fileActions.spec.ts

@@ -129,7 +129,8 @@ describe('registerHandler registry', () => {
 		const warn = vi.spyOn(logger, 'warn').mockImplementation(() => {})
 
 		registerHandler(makeHandler({ id: 'dup' }))
-		registerHandler(makeHandler({ id: 'dup' }))
+		// Another handler, not the same one registered twice
+		registerHandler(makeHandler({ id: 'dup', tagname: 'other-app-dup' }))
 
 		expect(warn).toHaveBeenCalledTimes(1)
 		expect(warn).toHaveBeenCalledWith(expect.stringContaining('dup'))

__tests__/registerHandler.spec.ts

@@ -0,0 +1,42 @@
+/*!
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { registerHandler } from '../lib/handlers.ts'
+import { scope } from '../lib/scope.ts'
+import { logger } from '../lib/services/logger.ts'
+import { makeHandler } from './factories.ts'
+
+describe('registering a handler whose id is taken', () => {
+	afterEach(() => {
+		vi.restoreAllMocks()
+	})
+
+	it('quietly keeps the first when it is the same handler again', () => {
+		// Several copies of the package on a page each register the
+		// defaults: that is one handler, not a collision
+		const warn = vi.spyOn(logger, 'warn')
+		const first = makeHandler({ id: 'repeat', tagname: 'oca-viewer-repeat' })
+		registerHandler(first)
+		registerHandler(makeHandler({ id: 'repeat', tagname: 'oca-viewer-repeat' }))
+
+		expect(scope.handlers!.get('repeat')).toBe(first)
+		expect(warn).not.toHaveBeenCalled()
+	})
+
+	it('lets a second copy of the package register the defaults without a word', async () => {
+		// The server registers them on every page, and an app bundling its
+		// own copy for older servers asks again
+		const { registerDefaultHandlers } = await import('../lib/defaults.ts')
+		registerDefaultHandlers()
+		vi.resetModules()
+		const second = await import('../lib/defaults.ts')
+		const { logger: secondLogger } = await import('../lib/services/logger.ts')
+		const warn = vi.spyOn(secondLogger, 'warn')
+
+		second.registerDefaultHandlers()
+
+		expect(warn).not.toHaveBeenCalled()
+	})
+})

lib/handlers.ts

@@ -211,8 +211,17 @@ export function registerHandler(handler: IHandler): void {
 	validateHandler(handler)
 
 	scope.handlers ??= new Map<string, IHandler>()
-	if (scope.handlers.has(handler.id)) {
-		logger.warn(`Handler with id ${handler.id} is already registered.`)
+	const registered = scope.handlers.get(handler.id)
+	if (registered !== undefined) {
+		// Every app bundles its own copy of the package, so the same handler
+		// can be registered more than once: the server's copy and an app's
+		// both register the defaults. The first one stays, as the custom
+		// element its tagname names is the first copy's too.
+		if (registered.tagname === handler.tagname) {
+			logger.debug(`Handler ${handler.id} is already registered, keeping the first registration`)
+		} else {
+			logger.warn(`Handler with id ${handler.id} is already registered for <${registered.tagname}>, ignoring the one for <${handler.tagname}>.`)
+		}
 		return
 	}