Yeah with EVNEW just create a new file, your best bet for duplicating a ship is to do the following.
Starting tip: Hover your mouse over each entry box, a tooltip will pop up explaining a little of what that entry is for. Very useful.
Starting tip 2: Have the nova reference spreadsheet open so that when you come to assign outfits and weapons and whatnot you can look them up quickly.
1: Open TWO copies of EVNEW, and a copy of notepad.
2: Create a new plugin in one window
3: Open the data files containing the ship's information in the other window.
4: Alt-tab between your plugin and the opened data file while copying the data over, do this as and when necessary to copy info over.
5: Write down some details EVNEW will give you (shan id, target pict id, shipyard pict id, shipyard desc id, escort desc id) while making your ship, and while digging around in the nova Data file, export the shipyard screen pictures by double-clicking on each instance of the ship you want (in your case, IDA Frigate) and going up to File > Export. Put them in a "graphic resources" folder or the like for ease of reference. Name them whatever you want. Note: You do NOT have to export the rle8 and rleD files unless you intend to edit them and add the edited ones in. You'll see why later.
6: In your new plugin, import the graphics by going to the relevant resource and double clicking on it (in the left pane) to create a new resource. Put the relevant id number in (if EVNEW tells you your ship target pict id is 3287, then put that number in the little id box, then go up to File > Import, select the pic you want to import then hit "Open", it'll display the pic, then hit OK, and the ship now has a target picture.
7: Do the same for each graphic resource and give it the relevant id number, then start copying over the shan data into a NEW shan file. This includes Image IDs and Mask IDs, you can just point the shan to another ship's graphics if you want, so if you wrote down all the details in notepad, go ahead and add them in now. If not, alt-tab back to the nova data window, grab the shan info, and put it all in.
8: Adding a Desc for the ship is easy too, just double click "desc, put in the ID number the shan section gave you (for the example ship, its shipyard desc ID is 13287 and its escort desc id is 14287.) then enter the text in the bigger box. Be warned, EVNEW sometimes likes to chew off the end character of a desc entry so add a couple of periods at the end of the description. the n linebreak method doesn't work in the descriptions either (only works in names in the outfit and shipyard screens apparently), and too much text and you won't be able to read it all (as descriptions for the most part in the outfitters and shipyard don't seem to be scrollable).
9: ...
10: Profit.
That's how I did mine.
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