Mr. Umakanthan has given a very correct and practical mahamantra. He has vast experience in practically resolving IR matters and has also suggested precautions. I wish to add here only two small points. When you want to control something you do not want, or you are resisting a new event from happening, and suppose it has taken shape, then the event (Union) comes up with double/triple force, and at that time, a lot of negative things and offshoots also take place. You must have heard a film's dialogue, "Jab jaljalla aata hai to kuda karkat saath lata hai."
The second thing I wanted to share is that suppose you are successful in resisting the formation of an internal Union for a few months or years; your workers will soon form it with the support of external members as office bearers in the executive body. They have little empathy, a tendency to push hard unreasonably, and keep pressuring you continuously. This is my viewpoint. In fact, as a consultant, in similar situations, I suggested to one of my clients not to resist forming a union, but the local management was not receptive. Eventually, CITU entered, and in that region, CITU had never been present before.
MBA syllabi cover IR and labor laws subjects. What you learn with proactive approaches through common sense and practical experience is not available in books. IR is a different dish; its taste is unique, unlike the tastes described by engineers, CAs, teachers, etc. I also recommend, based on experience, that your previous successes and strategies won't guarantee you the same results.
All the very best,
Regards, RDS Yadav