We very often see the same username created across many instances

Guilty as charged. I’ll say though, there are several legitimate reasons why one might want to do this. I personally use it as a substitute for Reddit’s multireddit feature, by grouping community subscriptions across different instances by theme. As long as users use the same username across instances I don’t think this practice should be automatically regarded as an attempt to sockpuppet. It that was the goal, the accounts would definitely not be using the same username across all the instances.