Hi @mide88 ,
It’s best to watch the Lesson again.
He mentions this specific thing, starting from ~9:22 (but like I said, do yourself a favor and watch the whole Lesson again!).
There are some other tricks to find ‘double Vertices’, but in general it’s best to simply do a Merge By Distance ever so often, at latest when something weird/unexpected happens (Select everything with A and then M > Merge > By Distance).
