i`ve been asked to install a junkers hardwood floor in an old house. top floor onto the top of the original pine floors. plan was to nail but was worried as there is what looks like water damage on the original pine under the window. moister readings are ok. the original floors have not cupped except in this area, have shrunk though. (after 200yrs). it`s my parents house and just want to know if it would be better to clip as i normally only deal with new builds.

Before you do anything I would be sure the cause of the water damage is solved. The moisture readings may be o.k NOW , but that can change in a hurry if the problem is not resolved.

IMO Junkers clip system is a pain, but that's just me. I would prefer to nail.
The direction of the new planks should run perpendicular to the existing. If this direction is not preferred I would sheet out over the existing, whether you nail or clip.
Either way you should solve the moisture problem 1st, and sand the cupped boards flat.

thanks for the help, turns out the damage was from the 60s. all of the houses experienced this, original hand made sash windows had rotted to dust by this time. just to be safe laid on a vapour barrier and clipped with a 18mm ply base to compensate for the levels. the walls ran out by up to 175mm over a 6m hallway. not one straight line int the entire house, found out latter on that the doors/windows were made to measure in the workshops (now where the garages are). i`ll post a picture of the street/house. to be honest if i had the skills this house should have a parky floor, mahogany/panga panga would look good.