There can be multiple reasons why delivery rates are not 100%. Some common cases include users having sporadic access to network or being inactive for a period of time.
Messages that can be delivered with WhatsApp will have a very high delivery rate. However there are many reasons why a message may not be delivered. You will have access to the exact status of a message by monitoring your webhooks. This is different from sending messages with SMS, for example, where you do not have access to final delivered status and resending the message may indeed produce a different outcome.
Messages may remain undelivered because a user's phone is out of service, or battery, or they have lost it and are getting a new one and have disabled their SIM.
If and when a user does reconnect to the network all the messages you sent will be delivered to them. Receiving more than one message with the same content will be a bad experience for them. They will be more likely to block you or complain. You will be more likely to be banned.