bouncing ideas off (of) each other

It's also a very intuitive way to organize your thoughts, since mind maps mimic the way our brains think—bouncing ideas off of each other, rather than thinking linearly.

Source: https://lifehacker.com/how-to-use-mind-maps-to-unleash-your-brains-creativity-1348869811

I’d like to know if the preposition “of” before “each other” is redundant.

It's not needed.

I recommend that all learners avoid using "off of". It's never necessary and, in BrE at least, can make the speaker sound poorly educated.

The "of" sounds a bit sketchy in AmE too.