As stated in the title, I do not understand how to manage in order to have removed songs back in library catalog.
Should I every time remove the synchronized folders and restart from the beginning?
No other simpler way?
There is no need to re-add anything. If the folder is still included into the library, a restart of WMP will reload all files within automatically. If not, go to Organize -> Manage libraries -> choose the library you want to organize, and add an appropriate folder.
If I understand correctly, you deleted something from WMP with the option, "Remove from Library Only" and now want it back in the library?
If so, you can simply restore deleted items as shown in the snip below.
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