There are many reasons to develop open source software – not only to create some necessary tool, but sometimes also just to build something. So this is not necessarily a case of wasted effort. Phrasing it like that would be highly disrespectful.
For sure, pointing them to your existing project would be a nice thing to do. Maybe you can get inspiration from their code, or they can incorporate some of your components. I closely watch “competing” open source projects, and point users to them if my project doesn't have some feature that they need.
But do not expect the projects to somehow merge. Open source is not a competition, and nobody wins or loses. Everybody wins if you cooperate, but nothing is lost if you don't. Instead of suggesting that all development concentrates on one of the projects, you might instead be able to identify some components that can be shared and maintained together.