PayPal does complete to next transaction step

-- HikaShop version -- : 2.3.4
-- Joomla version -- : 3.3.6
-- PHP version -- : 5.6.2
-- Browser(s) name and version -- : Mozilla/5.0 (Windows NT 6.3; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0

have been having trouble with the standard PayPal module/work flow; some customers complete, others can not complete Initially did not read forum and after upgrading to HS2.3.4 but HAVE now upgraded to latest version update but still experiencing issues.
Have checked with the server admin and PayPal to see if can see connection or SSL issue but non report. Have switched on error tracking on the PP module but am not receiving any input entries to the log file(even for completed transactions)
We and customer receive creation notification email and at back notification/order is created. but then it looks as if all action stops as the message completes to advise the item is already paid for

desperate now as I cannot find the reason why it does this. the site is live and customers not happy as it will not complete their orders

Suggestions and input much appreciated i

Thank you Nicholas

Had already increased the counter as I rebuild the site; but did look again at the PayPal settings and found that I had missed the "allow'' duplication of order number. Have now changed this and after checking now functioning appropriately

Thank you for your input