You're misreading the psionic weapon feat. You only get the damage bonus if you lose your focus, so you can't get it again until you focus is regained (DC 20 Concentration check, full-round action or move action with the psionic meditation feat). So a 7th level soulknife could do an extra 2d8+2d6 damage with his first attack only. Assuming he has psionic meditation, he could spend the next round recharging his mind-blade (a move action) and reaquiring his psionic focus (a second move action, success not guaranteed), and then get the bonus damage again on round #3.