Hello,
You can test something in order to check if there isn't another product with the same alias, for this go to Mass Actions (in System dropdown create a new mass action).
Reproduce this configuration with using your concerned product alias, and click on Process (on the top of the page, below Mass Actions title)
You will see if there is already product with the
same alias, because when you delete a product or a category the alias must be remove as well.
If
yes, you have to edit this (or these alias) in order that each
alias is unique.
Regards