Not sure why so many people here comment that your communication style is vague.
Both instructions and issue are clear. Send product after notification was sent via mail. Colleague did that and aso sent the mail again, which had already been sent.
Why people are talking about the product being sent as the issue in thus scenario is beyond me.
As for a solution: Let them repeat back to you what they're supposed to do in their own words to verify you're both on the same page, before the do what they need to do.
If you have tried this unsuccessfully, I have no further suggestions without a whole lot more detail except for: ask theco worker in question how they would have phrased the task if they had given it to someone else. Try and learn what their style of communication is and adjust for that particular colleague.