Can alt text be the same for multiple product images?

There can be more than one path here, and it depends on a couple of factors:

1. Do you want all the images to be competitive in image search, or are you okay with only your main product image being competitive?

2. Are those zoomed-in images crucial for individuals with accessibility challenges, or is it okay if screen readers and other such technologies skip over them altogether?

One path you can take is to give your main product image strong alt text, and the rest of your related images minimal alt text with slight variations:

Main image - "Widget by Company, the thing that does some things."
Zoomed-in image #1 - "Widget, view from the front."
Zoomed-in image #2 - "Widget, another view."

Another path is to give your main image a great alt tag, and let the others have duplicate alt tags. The reason this is not recommended, as you mentioned, is that it may not be great user experience for screen readers, and might devalue those secondary images for search, but technically you won't be penalized, so it's okay to do this if it fits your particular case.

If those supplemental images are not crucial at all, you can leave your alt tags blank, as if those images were design elements. (Web designers leave images like lines, geometric shapes, and background gradients without alt text, which makes them virtually invisible to screen readers and mostly ignored by search engines.)

Here's a related discussion on Moz's boards.

Alt text should be written as if it were describing the significant parts of the image to a blind person.

So yes, three images can all have the same alt text, but only if all three images are identical.

If the images aren't identical, the alt texts should include a description of their differences.

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Someone using the thee example alt texts (from another answer):

• Main image - "Widget by Company, the thing that does some things."
• Zoomed-in image #1 - "Widget, view from the front."
• Zoomed-in image #2 - "Widget, another view."

would think:

• Main image: "So that's what the widget doesn't look like!"
• #1: "So that's what it doesn't look like from the front!"
• #2: "So That's what it doesn't look like from somewhere other than the front!"

These alt texts are very inadequate.