Many times in a doubles match, when all four players are at the net, you finally get an easy volley and you end up hitting it right back to your opponent. This error could be a result of a habit you have developed with your practice partner. In practicing your volleys with a partner, you volley the ball back to the partner so that you both will get a better workout. This habit can not be avoided, it is almost a courtesy.
When the match starts, try aiming at one of the three short-Ts. These "Ts" are all on the service line, two outside "Ts" and one center T. The outside "T" is formed at the intersection of the singles lines and the service line and the center "T" is formed from the intersection of the service line with the center service line.
In match play, aiming at these three targets will really benefit you. Remember, you are hitting the ball by two rackets when you aim at the center "T". Hit this volley a little harder. You have a larger volume of court here if you over hit the service line.
When hitting to the two outside "Ts", remember that these are much smaller, pie-shaped areas of the court without a large volume of area, hit softer to these "Ts" with more accuracy. In addition, you are hitting the ball by only one racket, not two like when hitting to the center "T". You will gain more control by hitting slower here. Typically, you might know when to hit the ball here, but may make too many errors over-hitting. Many times your opponent will be coming back into position from the outside "T", thus, they will not be able to change directions so quickly. This is simply hitting behind the player. Try this the next time you play. It will make you more competitive.
Good luck on the courts!