Some users asked to log in again to post a comment

### Impacted plugin

Jetpack

### Quick summary

When some users try to post a comment, they are required to log in again to post it, even though they are already logged in.

> Are you [User Name]?
You are being asked to login because [user email] is used by an account you are not logged into now.
By logging in you'll post the following comment to [Site Name]:
[Comment text]


### Steps to reproduce

1. Log in as one of the affected users.
2. Go to a blog (owned by the same user or another).
3. Open a post and try to post a comment.
4. See issue.

### A clear and concise description of what you expected to happen.

Since the user is already logged in, the comment should be posted.

### What actually happened

User is presented with a login screen, asking them to log in again.

### Impact

Some (< 50%)

### Available workarounds?

Yes, easy to implement

### If the above answer is "Yes...", outline the workaround.

Workaround: enter login credentials to log in again.

### Platform (Simple and/or Atomic)

Simple

### Logs or notes

Context:

8655370-zen
8680786-zen

The issue names Jetpack comments on Simple sites but no files or tests. Start by reproducing the comment flow with an already logged-in user and inspect where the second login prompt is triggered. Done means affected users can post comments without logging in again, with regression coverage for the reported flow.

Written by the indexing model from the issue text.