19.0
InDesign will open the same file multiple times. In the past, if you have a file open and accidentally double-click the file in the finder to open that same file, it'll just open the window that is already open. In the 19.0 update, it'll open multiple versions of the same file (only if the file is from a version pre 19.0 – pre-converted), which can create issues if changes are made to one file (and they aren't updated in the other). Version control nightmare!