Sent mail reviews says I've sent more mails than I did

Hi people!

When I used my e-mail today, I sent an application letter to a certain adress.(using the Mac OS mail app, via Google Mail)

When reviewing later the sent mails section in the mail app, I see two extra mails in the e-mail, of former application letters I sent! Of course I was a bit scared that my application letter ended up in the wrong mailbox, or sth like that.

However, when reviewing via Safari Browser my google mail account, it says I only sent one mail, the correct one.

How is this possible, and how can I prevent this from happening in the future?

Thank you for your answer.