On Bricklink you can check the "Non-Unique" from your 'my inventory' then you will get all items that are duplicates. If a duplicate is found it is possible that Used and New are shown, even if only one of them is a duplicate. I haven't seen an option like this here.
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I haven't seen an option like this here.
No easy way to merge them, by hand per lot.
Thanks much! "Non-unique" is an odd term