I got stuck at step 36 when it shows that I should have Record processed: 10 and Record Failed: 0, so I imported the csv again and got double the data. I don't think it P-9 to P-19 should exist. Did I do this wrong? How can I delete an import/solve this problem?

Yes, that's a mistake and there's not an easy way to undo it so you'll need to unisntall Max's Pitch Package and resinstall it again and then do the import, once, again.
Got to Setup > Apps > Packaging > Installed Packages > and for Max's Pitch Package cilck uninstall and follow through the rest of the steps to delete it and all the data in it.
Then, you'll need to install again just like before and you'll have to pay again but I can refund you for the 1st payment. Send me an email to remind me at tim.hill@themaxlabsproject.com after you install again, ok?