When I was taught typing at school, we were made to type sentences with two spaces after the full-stop. Opinion has historically been split on this practice but most sources these days are clear that one space is now the only correct option (see 1st source below for a study that asserts 2 spaces promotes faster reading). Recently I have annoyed people at work reviewing my documents, as they have to replace my habitual two spaces with one. The problem from my end is that for nearly 30 years, most of which I have been touch-typing, I have consistently used two spaces. That’s a lot of muscle-memory to overcome!

Some random sources I found from Google: