Currently in Grist, a duplicate of a table is another view of that table, thus deleting the original source of the data deletes views of that data as well. We have it on our product roadmap to improve this experience by collecting raw data views which are the true source of data in a document.
To create another table in your document (and not a view of an existing document), click on “Add New” in the top-left, then select “Table” and “New Table.”
The trash currently collects deleted documents, and not tables within those documents. I can see why you’d like to retrieve deleted tables from the trash as well and have made a note of this as a feature request. Thank you!
In the meantime, you can restore previous versions of your document by looking at the document history which periodically takes snapshots (automatic backups) of your document. You can restore to the version you want. Here’s a guide in our help center: Automatic Backups - Grist Help Center