I tried all ways to get 7zip and Winrar to recognise any part including the first as a valid file and because the other attempts to open or extract the files had failed I never tried this part in the 7zip menu:- Extract to "DoubleR Pack\"
until now and it worked great.
The reason that I used hjsplit was because all the other methods I tried had failed, I often download files that need joining with hjsplit to make a complete archive for extraction with Winrar etc, it worked in this instance but I now also know about the method you used with 7zip.
I don't use 7zip very often as Winrar or Winzip covers 99% of my needs but it didn't in this case. Well you learn something new everyday
So to extract the files with 7zip, all you have to do is select DoubleR Pack.7z.001
and right click the mouse and under the 7-zip menu select Extract to "DoubleR Pack\"
if you don't want to install 7zip, you could use the hjsplit method. You can download hjsplit from lots of places just use Google hjsplit is a free and handy program for splitting files and joining them up. To use it for joining something like this, just start it and select Join, you then need to select the file that ends with 001 (DoubleR Pack.7z.001) in this case, you can give it a different name for the output file or just let it come out as DoubleR Pack.7z
the program will then join the parts together and you can then use Winrar to extract the files as you would normally.
You don't actually install hjsplit, its a program that you click on whenever you wish to use it, very handy at times and essential for some of the larger files that I have downloaded, mind you 7zip is also handy for some files and I know that a couple of 7-zipped games I had would not work if I used Winrar to extract them, they worked perfectly when extracted with 7zip though.