New user wanting to set up LR on windows based PC

Using a separate SSD for the LrC catalog(s) is generally recognised as the best arrangement. Personally, I wouldn't bother using a separate drive for the actual software installation. Unless you have very serious space issues on the system drive, I'd just go with the installation on the standard system drive.

LrC doesn't use a scratch disk, it uses the system/user temp folder (which you can, I think, relocate elsewhere). The usual guide is to ensure that there's at least 20% free space on the volume containing the temp folder.

What do you mean by "duplicates"? LrC will happily detect duplicates when they really are "file" duplicates, i.e. same file-name, same file-size, same capture date. It does not detect "image" duplicates, e.g. cropped exports of an image (so different file-size), or renamed exports (so different file-names). If you are dealing with "image" duplicates, if you can deal with them before import then so much the better....but if not there are LrC plug-ins I believe which make a reasonable stab at detecting these.