I am using two logos in one page and my code is
<img src="img/logo/logo-black.png" alt="logo" class="logo-white"/>
<img src="img/logo/logo-black.png" alt="logo" class="logo-black"/>
The validation basically says this:
Your HTML IS validated from a technical point of view, no software will encounter problems when parsing it, but having duplicated
ALTs have a very high chance of not accurately representing your pictures and which would then lead to an incorrect information when the image can't be loaded, or incorrect interpretation of your images by screen readers.
But still, if you're 100% sure your images are the exact same, having the same
alt text should be nothing but fine.