No player going in the top 5 let alone number 1 over all would ever stay in school for just a second year. The players that consider staying in school are those that will graduate if they stay. It makes zero sense to do a sophomore season if you aren't going to stay the full 4 years and you can be making millions by leaving.
Marcus Smart literally just did that last year. Cody Zeller did it the year before that.
Its not uncommon at all.