How to remove duplicate nnimap servers?

[Alice Bob]

Don't why there are two exactly same servers in 'server' buffer
list.

It's like this:

```
{nnimap: gmail} (opened)
{nnimap: outlook} (opened)
{nnimap: gmail} (opened)
```

The frist and third line points to the same Gmail account. I can
verify this by RET to check their mail forders.

I trid to execute 'k' to kill the third one. But Gnus reminds:

,----
| gnus-server-kill-server: Read-only server nnimap:gmail
`----

How to remove either one?

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