Old event listeners not being removed when scripts are updated

This issue has been experienced by two customers in recent months but we are unable to reproduce it reliably.

Issue

Customers are experiencing an issue where, after some unpredictable period of time, event listeners seem to somehow duplicate. When this happens, an older version of the event listener will trigger when the event fires, along with the most current one. If the listener is disabled, the current version ceases to run, but the old handler script continues to run.

Investigation

What seems to be happening is that an event handler will work as expected at first, then after some unknown period of time, probably at some point when the script has been edited, it begins triggering twice - one of which is an older version of the handler. This suggests that the handler is not being unregistered.

Running the following script on a customer's instance which had the issue only returned a single result:

import com.onresolve.scriptrunner.confluence.ConfluenceEventListener
import com.onresolve.scriptrunner.runner.util.AOPropertyPersister
import groovy.json.JsonBuilder
import groovy.xml.MarkupBuilder
def items = AOPropertyPersister.loadList(ConfluenceEventListener.AO_PROPERTY_KEY) as List<Map>
def writer = new StringWriter()
def builder = new MarkupBuilder(writer)
builder.pre {
    mkp.yield new JsonBuilder(items).toPrettyString()
}
writer.toString()

One possibility is that a race condition is occurring when Confluence tries to unregister the event handler. This would explain why it's affecting so few customers.