We have a huge task list (about 11,000 contacts) that we would like to convert into tasks. When we tried to upload all of these in one go. It did not push through and it seems it is just loading forever. Now we tried staggering the upload and limit the task list into 1000 per upload and it works but now everytime we upload another csv file (on the same project) it duplicates the custom fields that we have. I want to know if we are uploading it wrong?
Hey @MVictor welcome to the Asana community forum 
To avoid to get duplicate custom fields you have to ensure that the data in each CSV file exactly matches the existing custom fields. You can read more about that here: Asana CSV Importer • Asana Product Guide
To achieve this you could for example go to the project to which you want to import the contacts, then export the project data per csv, remove the whole content. Then you have the custom field names in the proper format exactly matching your project.
This way you can easily copy over the data to be imported.
Let me know how you go 
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