Two eggs in one day, and now soft shells

When you say they are free range, do they also have layers pellets as their main feed?  If not that could be the problem as well as them being ex-batts they could be coming to the end of their laying lives.  Best to give them mainly layers pellets so they have a balanced diet and access to crushed oyster shell for extra calcium which will help with shell quality.

If they have not been wormed recently it is best to do so with flubenvet as worms can also account for poor quality egg shells.

Keep an eye out for any other symptoms in case they are unable to lay a softie.  If they have access to crushed oyster shell and the problem persists you could try giving them some limestone flour.  They would only need a small amount.  Feeding dark green leafy vegetables will also help them to absorb the calcium they need.

It is possibly that they are near the end of their laying lives and things sometimes start to go haywire in the laying department.  If they are releasing 2 yolks at the same time and not producing a double yolker then it is difficult for them to produce a shell for both eggs.