Ignore the error when setting the user id twice to the same value (#321)

## 🎟️ Tracking

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## 📔 Objective

The mobile clients will call `init_crypto` multiple times on the same
client if the user types the wrong password/pin.

The simplest fix here is to make it so that `init_user_id` only errors
when there is an attempted UUID change, and trying to set the same UUID
is just ignored. The UUID will only be set the very first call either
way.

Ideally we'd do as mentioned in #278, and just provide this ID during
initialization once we have a pure/user-bound client split, but that's a
bigger API break.

## ⏰ Reminders before review

- Contributor guidelines followed
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- Written new unit and / or integration tests where applicable
- Protected functional changes with optionality (feature flags)
- Used internationalization (i18n) for all UI strings
- CI builds passed
- Communicated to DevOps any deployment requirements
- Updated any necessary documentation (Confluence, contributing docs) or
informed the documentation
  team

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  issue and could potentially benefit from discussion
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Author: dani-garcia

crates/bitwarden-core/src/client/internal.rs

@@ -184,7 +184,16 @@ impl InternalClient {
 
     #[allow(missing_docs)]
     pub fn init_user_id(&self, user_id: Uuid) -> Result<(), UserIdAlreadySetError> {
-        self.user_id.set(user_id).map_err(|_| UserIdAlreadySetError)
+        let set_uuid = self.user_id.get_or_init(|| user_id);
+
+        // Only return an error if the user_id is already set to a different value,
+        // as we want an SDK client to be tied to a single user_id.
+        // If it's the same value, we can just do nothing.
+        if *set_uuid != user_id {
+            Err(UserIdAlreadySetError)
+        } else {
+            Ok(())
+        }
     }
 
     #[allow(missing_docs)]
@@ -248,3 +257,27 @@ impl InternalClient {
         EncryptionSettings::set_org_keys(org_keys, &self.key_store)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::Client;
+
+    #[test]
+    fn initializing_user_multiple_times() {
+        use super::*;
+
+        let client = Client::new(None);
+        let user_id = Uuid::new_v4();
+
+        // Setting the user ID for the first time should work.
+        assert!(client.internal.init_user_id(user_id).is_ok());
+        assert_eq!(client.internal.get_user_id(), Some(user_id));
+
+        // Trying to set the same user_id again should not return an error.
+        assert!(client.internal.init_user_id(user_id).is_ok());
+
+        // Trying to set a different user_id should return an error.
+        let different_user_id = Uuid::new_v4();
+        assert!(client.internal.init_user_id(different_user_id).is_err());
+    }
+}