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When the mouse is clicked, mouseClicked() (or mousePressed()) runs. Say you do have a check - somehow - for a double click, but this click just so happens to be a single click. What do you need to remember so that - should another click happen in the next 250 milliseconds, that second click can be detected as the second click of a double click? You will need to know when that first click happened, and maybe where as well. Do you have enough global variables to store that information? Post your code - or a working trimmed down version of it - for more help.
First line of code say time1stClick = millis();