Are they actually the same email being retried or are they similar emails, e.g. an email for the first reply in a topic, then later an email for a second reply?
Where are you seeing the additional emails? Discourse logs, your email provider logs or somewhere else?
If it’s your email provider, depending on the delivery failure, that might be perfectly normal - temporary failures will be retried periodically for an amount of time before eventually bouncing, how many times and over what period will be entirely down to configuration by your email provider.
Do you have bounces set up correctly to be delivered to Discourse, either by having bounce emails reaching mail-receiver or using webhooks?
Ordinarily if Discourse is notified of these failures, the bounce score for a user will build and past a certain threshold (bounce_score_threshold setting), Discourse will stop trying to send emails to that user.