Same Page with two different URLs

Hello, there,

I’m currently new to SEO and came across the following: a website renders the exactly same page in two differente ways:

www.example.com/folder1/subfolder1 with canonical set to www.example.com/folder1/subfolder1
www.example.com/folder1?cat=902 with canonical set to www.example.com/folder1

This is probably hurting the SEO since it’s sending link juice to different pages even though they render the same page, right ?
The thing is, how bad can it hurt the SEO ?
I believe the best would be for both pages to render with the same URLs and with the same canonical, but it’d take us a reasonable amount of time in order to reconfigurate the site’s architecture.

With best regards,
Lucas