@robertj - welcome to the community from a fellow user

this is a good use case.. but I am not sure about your specific request/question in this case.. my interpretation is as follows, so please correct/amend as necessary:

• you have email addresses already of the individuals
• you are going to send them a unique code to use if they go to the typeform and complete the information
• you want to be sure that they don’t come a second time to the typeform and complete it to get a gift sent to them - either with the same email/code combination or a different email address but the same code

if I am correct in the above, you could do the following, without resorting to using the APIs:

• use a hidden field approach to add the unique code number to the url that you are sending in the email to your clients, suggest ensuring that the actual url address is ‘hidden’ via the <a href….> text </a> syntax in email :
  • syntax for the url to be sent in the email would be something like this: https://yourtypeform.typeform.com/to/sjvsXIYH#email=XXXXXX&giftcode=XXXXX   (replace the typeform name and replace ‘giftcode’ with the appropriate variable/field name you want)
  • enable hidden fields in your typeform to receive this information coming in from the url
• in your typeform, you can now ask the user to complete the form including asking them to enter their email address along with the special code
  • you can use the advanced logic features to compare the hidden variable email address against the user entered email address to confirm that they are the same - this is a simple validation check but not a rock hard show stopper - if they are the same then you can progress them to the next question, if they are not the same, you can enter again or kick them out - 
    • this is a lightweight method for reducing sharing the link to a friend with a different email address.. some folks are just not savvy enough to check the url for hidden fields 
• set up the email notification method to the user and your ending page to reflect something along the lines of “thanks for your details. We will send you a confirmation email shortly about your free gift” 
  • this allows you to have the data input into something like a Google Sheet to do the comparison of the email and discount code to other entries already in the data set. 
  • you can use something like the Document Studio add-in to generate an output message based on the data input, to send either a positive confirmation email or to send an email to the user saying that “unfortunately we cannot send you the gift as your email and special code have already been registered as having been entered. If you believe this is not true or have not yet received your gift, please contact us at … yadda yadda yadda” 
• depending on your typeform plan level, you can also set up a limit on how many responses you are going to receive for your typeform ie close responses after 256 entries have been received. 

those are just a few of the thoughts (and some of the things i have explored) to try and make sure that duplicate entries are dealt with. 

There are probably other ways to do this and check entries based on using the APIs but that is beyond where i am in my typeform journey at this time. 

if you are embedding the form in your web site, you may be able to use cookies to control access as well. 

at this point, there is nothing in the Typeform environment that would allow you to upload the codes into typeform and have them validated on use in your form. 

hope these collected thoughts make some sense.. If i have completely missed the target, please provide some more details so that those of us who troll the community can provide some more focused assistance. 

cheers

des