The Legend Instances Duplicator tool in the Duplicators group of the Project Setup tool-set can help duplicate multiple picked legend instances on the current sheet to many other sheets at once.
After the button is clicked, it asks to pick one or more legend instances on the current sheet.
After one more legend instances are picked and the Finish button is clicked on the pick status bar, the following dialog will appear for choosing target views.If the OK button (with the green check mark) in the dialog is pressed, the picked legend instances will be duplicated accordingly into the chosen views. An information dialog will appear to show what have been duplicated to where.
If the Cancel button (with a red cross symbol) is pressed, the dialog will be dismissed without doing anything to the Revit model.