There is no facility to not let service get invoked more then once, but you can do some twicks to return error or something else and come out of service if it is already running. Look into your IS’s “Service Usage” page for hints. 
Use a mediation. That is, don’t invoke service directly, … SO publish the input to this service as a document, invoke the service thru a serial trigger.
#webMethods#Integration-Server-and-ESB#webMethods-General