Thank you for contacting us.
Normally when creating listings via Linnworks for both FBA and FBM you have the same Linnworks SKU but have different channel SKUs. We do recommend to have different channel SKUs, one for the FBM item and a separate one for the FBA item. This will avoid several issues as 'ghost' listings and resolves the problem with Linnworks not knowing that the item was switched to FBA. In this solution article is explained in more detail why we do suggest creating 2 different SKUs for FBA and FBM: https://help.linnworks.com/support/solutions/articles/7000025971-amazon-automatically-switching-between-fba-and-fbm
I hope this information is useful to you.
If you require further assistance, please, let us know.
Looking through the documentation it does not seem to be possible to list the same product with the same SKU as both a Merchant fulfilled & an FBA product without creating a separate SKU product which defeats the fact that its the same product, surely this is not the case, can anyone shed any light on this? thank you.