nesdev.org

The page http://wiki.nesdev.com/w/index.php/MMC5 documents the intended registers of MMC5 quite well.
Are any of these registers mirrored? Such as, does writing into $5170, $51B0, $5330, $5530, or $5930, achieve the same as writing into $5130?

I guess the answer is "we don't know, and you should test this yourself". As the answer with most advanced MMC5 questions

If you write a hotswap test ROM, I have a few MMC5 carts I could test for you.

I'm not sure about writable registers, but I know that none of the readable registers are mirrored - a number of years ago, I wrote a test program that tried to read from all of $5000-$5FFF, and the only locations which returned data were the known documented readable registers ($5010, $5015, $5204, $5205, $5206, and $5C00-$5FFF), with everything else returning open bus.

I'm just wondering whether you can safely read an address like $51E6 and not get side effects.