One of the possible errors you can run into with your eBay listings is this:
Errors: SKU already exists on market with IDs [IDs Here], will not duplicate
This particular error means the SKU you are trying to send to eBay already exists on your eBay marketplace. If you take the IDs provided and search for them in your eBay account you should be able to easily identify which SKUs are duplicates.
Deleting those SKUs from eBay and letting Listing Mirror push the entire listing out is a good way to resolve the issue. If you have sales history you don't want to lose by doing this, you can delete the SKU from Listing Mirror and contact support to import the existing SKU from eBay.
Please Note that it may be tempting to delete the SKU from Listing Mirror and recreate the item with another SKU, allowing yourself to have two identical fixed priced listings on eBay. This is expressly against policy and can result in action from eBay against your eBay seller account.
If you have any questions, please reach out to support.