Every time I open a task (Firefox, Dolphin, etc) it's opened once, but in the task bar I see 2.

e.g.: I open 1 instance of Mozilla Firefox, and opens 1 instance of Firefox (I check it with alt-tab) but in the task bar there are 2 mozilla firefox.

Anyone knows how I solve this ?

(I want one only one task in task bar per task I open)

thanks in advance for any feedback

best regards,