'JWCTurner' - This fix didn't work for me unfortunately. So as ever, every time I click on the Dock finder icon, rather than moving to an already open finder window, a new window always opens.
Apple - please fix this as it's such a commonly clicked item, how did you miss this?
Here's how I solved this problem for myself. I was opening folders in Finder and discovered that each folder was opening in a new tab. Very annoying!
Settings > General > Prefer tabs > in full screen [when opening documents]