To duplicate a field ?

Unfortunately, no.

Nick is right but if you mean that you just want to show that field & data twice so on the same table/form.

Then what you can do is have a forumula field that points to the original field.

Example :

On my Orders from - I have an order date field (OrderDate) that calculates todays date on creation. But I don't want users to inadvertantly change it in error. 

So I have moved this field onto an "admin" tab then on the front of the form I have a forum field (Order Date) (note the space in name) and in its formula simply have Orderdate

Users can't edit that date and I also get the benefit of being able to style that field. 

in addition all fields on my Admin tab page are all protected by being set to only display a a lock field is set to eg XXXX

at the top of the form I have the Lock field - for unlocking these fields

Works for me :-)

Also if you want to duplicate that field and all its data into another field on same table then you can do the following:

Example

I had a telephone field that all the leading zeros were missing (Telephone) on approx some 200,000 records

so constructed a formula field (fxTemp)

in that field I had formula "0" + Telehone

New field was called Telephone1

then created a button

and in the button the script was 

for i in (select MasterData)[Telephone1 = null] do
let a := i.fxTemp;
i.(Telephone1 := a)
end

That got me 2 fields and when happy I deleted the first field 

saved then and renamed the new field back to Telephone