This can happen when you build listings in Seller Central, and hop into InventoryLab to use them before they have pulled through to be recognized by the InventoryLab system as already created. We update this data every 3-5 hours in your account.
There are a couple of options here... You can choose to wait until that next sync happens before listing your items. When that happens, we will pop up a box for you recognizing the ASIN and MSKU you created with Amazon and you will be able to click a button on that popup to use it.
Realizing you may not want to wait that long, there is an alternative to prevent the duplicates where you can still list right away!
Search the item and enter in your sourcing details, as you would any other listing, and scroll down to the MSKU area
Highlight and delete the new MSKU that was automatically created for you
Copy the SellerSKU of your product from Seller Central and paste it in the MSKU area of InventoryLab
Complete your listing process as usual by clicking the Add to Batch button
This will make sure Amazon is using the listing you created instead of creating a new one under a different SellerSKU/MSKU and losing all of that valuable detail you put in.