This could be related to sudden very long http-request times at authorize.net for AIM single payments, and users refreshing pages or a timeout occuring before the payment completes (if the authorize.net side checks for double payments only after the other transactions http requests are completed,if you still follow me).
This was due to a server software change on authorize.net side that introduced a backwards incompatibility.
We have now worked around this and solved these very long requests. Full reply isl here:
www.joomlapolis.com/forum/255-developer-members-support/240719-7495-authorize-net-gateway-not-working-sorta?start=6#311342
In short, please update to latest nightly build, and see if issue persists. Don't fill-in MD5 hash or signature key (new param) as you are not using auto-recurring payment subscriptions.
Please let us know here if it fixes the issue for you. Many Thanks in advance.