Please check how you have recorded the email address for your 'original' contact inside your CRM account.
It is likely that the contacts email address has been recorded but not saved as an 'Email' type.
It could instead be saved as a 'Mobile' or 'Phone' type for instance - as shown in the example screenshot below:
This means that if you send the contact an email and copy that email into your CRM account then as the system could not find a match with another email address inside the database (as the email address could be recorded as a 'Mobile' number for example) so therefore it created a new contact record record - where the email address has been stored as an 'Email' type.
If this has occurred you will need to merge the duplicate contact into the 'master' contact record. Please click here to see more about how to merge contacts.