"Is this an issue with iATS payments Extension?"
No it's not an issue with the iATS Payments Extension.
I got this question - and have reproduced it using 4.7.7 and iATS Payments Extension [note the transaction IDs are identical]:
But I also get this on the http://dmaster.demo.civicrm.org/civicrm/contribute?reset=1 [today May23 at 9:40MT] - again: note the transaction IDs are identical:
I will file a JIRA issue for this - Please note that iATS Extension is officially still on the 4.6.x (LTS = Long Term Release).