Avoid duplicates in array

Avoid duplicates in array

I'm sure this question has been answered before but the answers I have found don't seem to apply.

Insert code: (pretend this is in the code box)

var arrayEngineSizes = new Array();
array1.forEach(function(entry) {
      if (entry.model == y)
 {
 var loc = $.inArray(entry.enginesize, arrayEngineSizes);
 alert(loc);
 if (loc == -1) // supposedly prevents duplicates, but not really, always says -1
   //arrayEngineSizes.push(entry.enginesize); 
{
arrayEngineSizes.push(entry);  
alert('pushed value containing ' + entry.enginesize);
//loc = arrayEngineSizes.indexOf(entry.enginesize);
loc = $.inArray(entry.enginesize, arrayEngineSizes);
alert('what does loc say now? ' + loc); // always says -1
}
 }

Yes, I don't know how to use the forum code box. I can never escape from it once I get in it.

Anyway, why can't I find and avoid duplicates using the code above, other than the code being bogus?