In volleyball, if the ball is rebounds from the net, can the player who hit the ball into net hit the ball twice, or is this a double hit?
Yes, this is a double contact. The basic rule here is 9.1.1 (all quotes from the FIVB 2017-2020 rules):
9.1.1 CONSECUTIVE CONTACTS A player may not hit the ball two times consecutively
Exceptions to this rule are detailed in 9.2.3:
9.2.3 The ball may touch various parts of the body, provided that the contacts take place simultaneously.
18.104.22.168 at blocking, consecutive contacts may be made by one or more player(s), provided that the contacts occur during one action;
22.214.171.124 at the first hit of the team, the ball may contact various parts of the body consecutively, provided that the contacts occur during one action.
As the contacts were not simultaneous, not part of a block and did not occur as part of one action, none of the exceptions apply and this is a fault.