Your first problem is that you are taking bass lessons from a guitarist. That’s like getting a race car driver to teach you how to drive a semi. They may be expert drivers, but in completely different ways with completely different vehicles. Back from my days as a bass educator, I would take in to account your age and your goals. If you are 16 and dreaming about music college then darn right I’m gonna teach you perfect technique and be a stickler about it. If you’re a 67 year old retiree and just wanna play a few tunes at your church then I’d meet you where you are as far as technique and help you achieve that goal, and then build from there and slowly build that perfect technique. There are no “nevers” in music. There are plenty of situations where repeating one finger is fine or even preferable. I recommend you find a new teacher, one that’s both a good bassist and a good teacher. Despite what they all think, bass, it’s role in music, and the approach to filling that role is not something guitarists understand.