duplicate workflows issue

hi all

I have requirement that i have to send out email to contacts but the issue is that

the entity contacts are attached to are multiple for each contact and this makes the user to

initiate the workflow multiple times for same contact. the challenge is i can only send one email 

for each contact. how do i know if workflow already started for each contact to stop duplicate workflows for the same contact?

if its confusing to understand please let me know to help you understand.