I guess you'd like to be able to select multiple fields and duplicate them. Please see how you can do it:
I have cloned this form of yours.
1. Select one of the fields that you'd like to duplicate. I have selected this one and proceeded like this:
2. You can select as many fields as you'd like on the opened pop-up window:
I have selected the fields that are displayed with a yellowish background. Then I pressed the "Duplicate" button to copy those fields.
Please try this and let us know if this is not what you are looking for.
Do contact us again if you need any further assistance.