Inconsistent behaviour when registering event listeners

[charltoncraig]

Expected Behaviour

Whether registering an event listener before or after adobe-client-data-layer.min.js has loaded, the callback function should fire just once

Actual Behaviour

When registering an event listener before adobe-client-data-layer.min.js has loaded, the callback function fires twice (fires just once when registering it after the library has loaded)

Reproduce Scenario (including but not limited to)

Steps to Reproduce

Navigate to about:blank in a browser
Execute the following in the console (registers the callback before the library loads)

window.adobeDataLayer = []
instance = 1

var myHandler = function(event) {
    console.log('Instance: ' + instance)
    console.log(event)
    instance+=1
}

window.adobeDataLayer.push(function(dl) {
    dl.addEventListener(
        "adobeDataLayer:event", 
        myHandler
    )
})

window.adobeDataLayer.push({'event':'newEvent','context':'test'})
scriptElement = document.createElement('script');
scriptElement.src="https://unpkg.com/@adobe/adobe-client-data-layer@1.0.0/dist/adobe-client-data-layer.min.js";
document.body.append(scriptElement)

(Optional) Execute the following in the console (registers the callback after the library loads)

window.adobeDataLayer = []
window.adobeDataLayer.push({'event':'newEvent','context':'test'})
scriptElement = document.createElement('script');
scriptElement.src="https://unpkg.com/@adobe/adobe-client-data-layer@1.0.0/dist/adobe-client-data-layer.min.js";
document.body.append(scriptElement)

instance = 1

var myHandler = function(event) {
    console.log('Instance: ' + instance)
    console.log(event)
    instance+=1
}

window.adobeDataLayer.push(function(dl) {
    dl.addEventListener(
        "adobeDataLayer:event", 
        myHandler
    )
})

Platform and Version

Chrome 83.0.4103.116

Sample Code that illustrates the problem

Logs taken while reproducing problem