See two EMAILS in thunderbird after checking Webmail (same account)

[Boma]

My boss sends an email using Seamonkey (Thunderbird engine) to himself. He then checks it in JUNO Webmail (same account).

When he checks the mail again in Seamonkey. He gets an extra copy of it in his inbox.

It's just a small annoyance, but something I wanted to help him fix.

Thank you all.