just thought I'd post my assessment for anybody who finds themselves in a similar situation

turns out I was correct in thinking this was a double submit issue,

I tested this by calling submit several times from the command line

 $('#form').submit(); $('#form').submit(); $('#form').submit();.... 

• the first submit goes through successfully
• the subsequent requests were lagging behind and now ignored by ajax handler but still firing on the network
• for the first 4 or 5 lagged requests, the integrity constraint violation fires
• for the remainder of the requests the validation kicks in and says the user already has used this email address (but they won't see this as we have already got past this stage from the first submit)

so it wasn't affecting my users, but it was causing unnecessary queries on the back end.

solved now by preventing form submit twice, unless there's an error callback first, at which point they can submit again.. which obviously I should have done in the first place ;)