fix: more lightweight and succint client-side err msg

docs/headingsDetached.tsx

@@ -367,6 +367,10 @@ const headingsDetached = [
     title: 'Client runtimes conflict',
     url: '/client-runtimes-conflict'
   },
+  {
+    title: 'Client runtime loaded twice',
+    url: '/client-runtime-twice'
+  },
   {
     title: '`includeAssetsImportedByServer`',
     url: '/includeAssetsImportedByServer'

docs/pages/client-runtime-twice/+Page.mdx

@@ -0,0 +1,16 @@
+If you get this warning:
+
+```
+Client runtime loaded twice
+```
+
+Then this means that the page has loaded more than one copy of Vike's client runtime. In other words, the client-side bundle of the page includes Vike's client runtime twice (or more).
+
+In order to reduce the size of your client, make sure that the client-side JavaScript of a given page includes Vike's client runtime only once.
+
+You can, for example with Unix, inspect where Vike's client runtime is included:
+
+```bash
+cd dist/client/
+grep -r 'loaded twice'
+```

docs/pages/client-runtimes-conflict/+Page.mdx

@@ -1,13 +1,16 @@
 import { Link } from '@brillout/docpress'
 
-If you get the following warning:
+If you get this warning:
 
 ```
-The client runtime of Server Routing as well as the client runtime of Client Routing are both
-being loaded. Make sure they aren't loaded both at the same time for a given page.
+Client runtime of both Server Routing and Client Routing loaded
 ```
 
-Then the problem is likely due to one the following.
+Then this means that the page loads the client runtime of <Link href="/server-routing">Server Routing</Link> as well as the client runtime of <Link href="/client-routing">Client Routing</Link>.
+
+A given page should *either* use Server Routing *or* Client Routing, never both. Thus only one runtime should be loaded.
+
+The problem is often due to one the following.
  - You configured [Rollup's code splitting](https://vitejs.dev/guide/build.html#chunking-strategy) in a way that includes both runtimes inside a same bundle. For example:
    ```js
    // vite.config.js
@@ -28,7 +31,7 @@ Then the problem is likely due to one the following.
 
    export default { build: { rollupOptions } }
    ```
- - You're importing a utility that is only available to <Link text="Client Routing" href="/client-routing" /> such as `prefetch()` or `navigate()`, for a page that uses <Link text="Server Routing" href="/server-routing" />.
+ - You're importing a utility that is only available to <Link href="/client-routing">Client Routing</Link> such as `prefetch()` or `navigate()`, for a page that uses <Link href="/server-routing">Server Routing</Link>.
    ```js
    // ❌ Don't import these for pages that use Server Routing
    import { prefetch } from 'vike/client/router'

vike/utils/assertSingleInstance.ts

@@ -25,9 +25,8 @@ const globalObject = getGlobalObject<{
 })
 
 const clientRuntimesClonflict =
-  "The client runtime of Server Routing as well as the client runtime of Client Routing are both being loaded. Make sure they aren't loaded both at the same time for a given page. See https://vike.dev/client-runtimes-conflict"
-const clientNotSingleInstance =
-  "Two vike client runtime instances are being loaded. Make sure your client-side bundles don't include vike twice. (In order to reduce the size of your client-side JavaScript bundles.)"
+  'Client runtime of both Server Routing and Client Routing loaded https://vike.dev/client-runtimes-conflict'
+const clientNotSingleInstance = 'Client runtime loaded twice https://vike.dev/client-runtime-duplicated'
 
 function assertSingleInstance() {
   {