To duplicate a form, click the Duplicate button at the top of the page:

You will be given a couple of options to either Copy as linked or Copy as unlinked. 

Copy as linked - the fields within the new duplicated form will be identical to those in the original form and they will be linked together. This means that any value you input into a linked field on the original form, will synchronise across to the other. 

Copy as unlinked - the fields within the new duplicated form will not be linked to the fields on the original forms, and as such, the values you input on the new form will not synchronise across to the original form.  This is helpful if you would like to create a form similar to another form you have set up.