Randomizing between 2 movieclips cannot prevent you from having the same movieclip play several times in a row.
Random between 2 and 3 could be 2,2,2,3,3,2,3,2,3,3,3,2,2.
What you need is a toggle and the only way to have one or the other to play would be to check a variable.
var on = true; _this.Play_button.addEventListener("click", playMC.bind(this)); function playMC(){ if(on){ this.gotoAndStop(2);
// play movieclip1 code on = false; }else{ this.gotoAndStop(3);
// play movieclip 2 code on = true; }
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