Why am I seeing two identical payments associated with one contribution in 4.7.x?
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).