getInitialData not called when leaving error route to the same component
If client-side navigation (`<router-link>`) is used to switch from the error page to an another page *but* both the error page URL and new page URL resolve into the same route component, `getInitialData` is not called due to false positive duplicate check in `router.beforeResolve` at `app/client-entry.js`

### Steps to reproduce

Create page component:

```vue
<template>
<div>
  <router-link to="/a">a</router-link>
  <router-link to="/b">b</router-link>
  <router-link to="/c">c</router-link>
  current page: {{ page }}
</div>
</template>

<script>
export default {
  getInitialData (context) {
    if (context.route.path === '/c') {
      context.router.app.setError({ code: 404, message: 'not found', errorPath: context.route.path })
      return
    }
    return {
      page: context.route.path
    }
  }
}
</script>
```

error component:

```vue
<template>
<div>
  <router-link to="/a">a</router-link>
  <router-link to="/b">b</router-link>
  <router-link to="/c">c</router-link>
  error: {{ error }}
</div>
</template>

<script>
export default {
  props: [ 'error' ]
}
</script>
```

and entry file:

```js
import Router from 'vue-router'

import test from '@/pages/test'
import error from '@/pages/error'

export default function () {
  return {
    router: new Router({
      mode: 'history',
      routes: [
        { path: '/:path(.*)', component: test }
      ]
    }),
    error
  }
}
```

Then open http://localhost:4000/a

### Expected result

Switching between /a, /b and /c works fine in any order and regardless of which page was generated with SSR at first.

### Actual result

Switching between /a and /b will work fine. Switching from /a or /b to /c will display error as expected. However, switching from /c to /a will crash:

```
[Vue warn]: Property or method "page" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

found in

---> <Test> at src/pages/test.vue
       <ReamRoot>
         <Root>
```

and the page info will be empty:

```
a b c current page:
```