Question

How can I check if a google map marker is already inside an array of markers?
Even after this markersArray.push(marker); the condition (marker in markersArray) is false.

Answer 1

First, (marker in markersArray) is wrong since in doesn’t look for elements in the array.
It looks for properties.

The way it worked for me was

for (var i=0; i<markersArray.length; i++) {
    if (markersArray[i].getPosition().equals(marker.getPosition())) {
...

This works as long as what you need compared is only the coordinates of the markers.
We use here the LatLng class’ .equals operator.