Currently using Odoo v15,

I have a catalogue of over 1000 products that I am trying to import into odoo. Some of these products have variants. I can import them fine, but the issue comes when I try to update them later.

For example, I have a t-shirt that has 2 colours: Black and White, and 3 sizes: S, M, L.
I now want to add the colour Blue to this product using the import feature. However, instead of adding the Blue to the original colour attribute, it will create new lines with Black, White and Blue as well as Sizes of S, M, L instead of updating the lines that already exist. 

I have tried using the attribute ID to make the changes but the duplication issue happens regardless.

Is there a way to update attributes on an already created product using the import tool?