A few years back I started my Shopify website. Like many other people, I resorted to creating a master product and copying it over and over (and over). The result - products got created but the 'handles' got mixed up. So if I had a product called 'red hat', it would be called mysite(slash)red-hat but if I copied it to a 'blue hat', and changed the name after saving the product, it would be called mysite(slash)red-hat-1 (aagh!) and if I changed 'red hat' to 'green hat' the handle would still stay as red-hat. Not very SEOish, right?
I then created a Shopify app called 'Handle SEO'. It worked so well for me that I made it available to Shopify merchants worldwide. The app got a good take-up and thousands of products are scanned every day to fix the handles. And the app creates automatic redirects from your old not so SEO friendly handle to the new SEO friendly handle.
Give it a shot. There's a low one-time $10 price with a one-day trial period, so you've nothing to lose. And if you like it, please leave a good rating.