Symfony version(s) affected: 5.2.3, but probably any from 4.1.x

Description
Localized routes that are prefixed with a locale prefix encounter a number of issues:

1. When all locales are prefixed, the base route is still accessible.
2. When all locales except the default are prefixed, the default route doesn't switch the locale back to the default langage.

How to reproduce
Say you have a controller method bearing the following annotation:
@Route("/shopping", name="shopping")

Issue 1. When all locales are prefixed, the base route is still accessible
This configuration:

shopping:
    resource: ../../src/Controller/Application/Shopping/
    type: annotation
    prefix:
        fr: '/fr'
        en: '/en'

Will give you 3 routes:

1. /fr/shopping
2. /en/shopping
3. /shopping which is undesired

Issue 2. When the default locale is not prefixed, the default route doesn't switch the locale back to the default langage
When you don't force a prefix for the default locale, as in this configuration:

shopping:
    resource: ../../src/Controller/Application/Shopping/
    type: annotation
    prefix:
        fr: '' # "fr" is the default locale here
        en: '/en'

It will give you 2 routes:

1. /shopping
2. /en/shopping

Which is the desired behavior. But another bug appears:

1. if you visit /shopping, the site is in french
2. if you then go to /en/shopping, the site switches to english
3. if you then go to /shopping, the site stays in english

Both A and B issues are undesirable (notably from an SEO standpoint):

• Issue 1: /shopping is an unwanted duplicate content of /fr/shopping
• Issue 2: if the bot that visits the website isn't stateless, it can be served english content on a french page (making it a duplicate too, and unreferencing your french page).

Additional context
When routes are declared using the annotation:

 * @Route({
 *     "fr": "/fr/shopping",
 *     "en": "/en/shopping"
 * }, name="shopping")

The issue 1 does not exist. There is no /shopping route.

Same thing goes for a route that wouldn't have the prefix for the default locale:

 * @Route({
 *     "fr": "/shopping",
 *     "en": "/en/shopping"
 * }, name="shopping")

The issue 2 does not exist. The locale switches correctly back to fr when visiting /shopping, as expected.

So it seems the issue lies in the way prefixes are implemented.