Take only one action ~ if(A = var[i]) reveals a true

Sorry noob question! I’m comparing a gameobject transform to all transforms in an array. if not equals to any values in array, i just want to take an action 1 time, i only know how to do it as many times as i do the comparison:

for (var marker in markers)
{
if ( pos = marker) //return true if 1+ value is same and do just one
{Instantiate(obj,pos,rot);}

With great help from @mikebelotti :

for (var i:int = 0; i < markers.length; i ++)
{
    if ( markers *!= pos )*

{
Instantiate(obj,pos,rot);
break; // fall out of the loop
}
}