If you use WordPress and Woocommerce, you might bump on this problem. SKU (Stock Keeping Unit) is a unique product code. Uniqueness is not optional by default, if have to be unique.
Let’s say you are selling t-shirts and you have colors as variations, so blue and green t-shirt are just 2 variations of exact the same product. In Woocommerce you will have to place different SKU for that two variations.
But what is you want to have exact same SKU for two variations. Or even for two different products. We don’t want to question your reasons for that.
Here is a nice free WP plugin you might use for that. “Neparno 10” developed it and placed on their website. Plugin is called “Disable unique SKU“. You will just need to upload it to /plugins folder and activate it. After that, you will be able to enter exact same SKU for different products and variations.