Others may have answered this already.
There are 2 versions of this. The first is when someone contacts you to sell you a version of your domain name. They may not have bought it yet. If you go onto Internic.net or a whois tool and enter the domain name, it will tell you whether it is registered or not. If not, choose whether you really want it and register it yourself.
If it has been registered, it is called domain squatting. Domain registrars don't look kindly on this behaviour. Use the whois tools to find the registrar and report it as domain squatting.
If you are concerned about this happening to you in the future, co sider registering a trademark. It will make the process easier. It will also force the squatter to hand over the domain to you. You can then choose to renew it after the initial registration has expired.
Consider registering the domains you care about at first, it's not really a big cost. You can always point them all to the same host. ( e.g. I own drone-films.nl and Drone-films.eu) which all points to the same host.