Copying one for your own use is okay, but copying a design to sell, well........
I built kites since 1994 and I'm on kite forums in Fr, UK, german, US and every single time somebody says he's going to built a copy of a kite somebody has to say just that as a warning/guilt trip... even for a copy of a 25 years old model.
Anyway, what model do you want to make, or what kind of kite is it and what is the panel design ? Any pictures ?
Usually to copy a kite, you remove all the structure and try to lay down the sail on a thin cardboard the best you can, holding it with tape and/or little weights. Even if the kite is brand new the sail won't lie flat (the stitches shrink the fabric), try not to stretch it too much in one direction or the other. If the stitches for each panel are straight you can try to stretch them.
What you can do is take the outline of the right side and the outline of the left side and the blend of both will be your final shape for the half kite, create the other side by symmetry. Work on the full sail and draw the panels at the end, don't try to copy each panel independently, they will never fit together at the end.
When you sail is finished, try to fit the structure of the old kite on it to see if it fits ok, sign that there is no major mistake and from there, adapt the structure to your sail, don't cut the bars before the sail is finished. Make sure the structure for each kite of the stack is the same.
Remember to take into account the hems and overlap before you cut your fabric.
For the assembly, use UHU stick glue if you don't have specific thin double sided tape to join the panels together before sewing. Even with a walking foot sewing machine (like the IDT system from old Pfaff), you need to glue the panels together before sewing. Remember, the smaller the stitches are, the bigger the needle/thread, the more the fabric will shrink.
Like it was said before, your copy certainly won't fly exactly like the original one you have that is stretch from years of use, so make your stack with your home made copies. Make sure you sew them with the same sewing machine settings to have the same kind of shrink/stretch when you sew the panels together.
I can't think of anything else that can be a pitfall right now.