Currently when I select a bit of text replacing another bit of text (eg "viwp" to paste over the current word) it copies the thing I just replaced into kill ring. So if I try to paste over another bit it has lost the original thing I copied and instead pastes the thing I previously replaced. Obviously I can cycle through the kill ring but that's fiddly and a total pain.
Any idea what variables to look at? I think this happened when I upgraded evil-mode.