MT5 supports Netting and Hedging accounting systems. The Netting system implies that you have one deal for one symbol. That is, if you already have an open position on some instrument, executing a deal in the same direction will result in an increase in volume of this position. Consequently, when you open a deal in an opposite direction, the volume of your position decreases. If the Hedging system is applied, opening a position on an instrument will result in opening a new deal. You can have several positions on one and the same trading instrument.