You can duplicate and save existing records by performing a copy & paste operation on the records. To duplicate records, follow the procedure below.
Click the record selection check boxes and select the records you want to duplicate.
By holding down the Shift-key and clicking the check box, it is possible to select / deselect records in a specific range at once.
Copy with Ctrl (Command in MacOS) + C from the keyboard and paste it with Ctrl (Command in MacOS) + V.
Since records with same information as the original are added as rows, edit them and press the "Save" button to save the records information to Salesforce.