Just to clarify, your issue is just with this single task?
I suspect you are running into the async executor’s 5 minute lock. The other async executors think the job has been killed without releasing the lock and start a new instance. How to change the 5 minute timeout interval for a async job?
You can either increase the timeout, to one that better meets your needs, or you can switch to using a triggerable service task and trigger the execution when the long running REST call returns.