I have an email address in the first To: field. I try to paste another one in the second To: (below) or Cc: doesn't matter, but the program pastes it over the one in the first To: field. The mouse cursor was in the second field before 'paste'. The Linux is LXLE (based on Ubuntu 14.04).
You are mistaken. You need to make the area where you enter addresses taller, either that or use the scrollbar that appears on the right after you've pasted in the 2nd entry. All that is happening is that the first address is automatically scrolling upwards out of sight, it is not getting replaced by the 2nd one you paste in.