Absolutely nothing is wrong with submitting the same pitch to multiple sites. Now, there are some potential issues! For example, if your pitch is very generic, you’re liable to be ignored as spam. I always recommend tailoring a pitch to the site you’re submitting it. However, you can submit a pitch for the same topic to multiple sites.
This is all perfectly fine so long as you do one thing. That one thing is writing unique content to each site that accepts your guest post pitch. I actually quite like this method, because when I’m writing about a topic, I tend to pull up case studies and research, and keep all of the information on hand. If I can write three different articles on the same topic using the same research, it’s a lot easier than writing three unique articles on three different topics.
Of course, you have to limit the number of pitches you send out to the number of times you can cover the same topic without looking like boring spun content or duplicate content. As long as you can make unique variations, you’re good to go.