Two workflow instances started

[Kiran, Gb]

Hi,

we have a workflow in production which is triggered by an event 'Changed' of Business object and is working fine for a year now. usually 30 instances of this workflow are started daily .

In some rare cases we have observed that two instances of the workflow are triggered by the event and this causes confusion amongst users and also in the event broadacst used in the workflow.

have any one of you have an idea 
1. the reason for this occurence 
2. where do we need to look for trouble shooting
3. any hints

Thanks and regards
KB