Limit workflow object instances

hi gurus,

apologies if this is a common question, I searched but found no answers, so perhaps it is not...

I want to limit the number of times a workflow can be started for an object.... example, I have a workflow for our material master creation process which is started manually. If there is an instance of the workflow in process for given material I do not want another user to be able to manually start it again for that same material.

Any clue on how to do this?

many thanks,

Neil