Woocommerce Product category base same as product base causes 404

You can not realistically do such a thing. WordPress URL parsing is just not “smart” enough to be able to handle different types of content under the same “prefix”. WordPress will either try to parse the URLs as product or categories, depending on the specific order they are found in the rewrite table, and will fail on the other type.

If you have to have such a scheme you can override the rewrite handling and implement one of your own (there are probably few examples on this site), but I think the work required and the complexity of code introduced by such a thing will not be worth it (the code should not be very complex by itself, but whoever will maintain the code after you will suffer a big WTF moment trying to understand what is going on).