You answered the question yourself. Preexistence of similar product is an indication of the demand. Given that your product have some additional feature, you should try to leverage the same and gain first mover advantage. Once you hit the tarmac you'll see other ideas evolving within yourself to maintain that advantage.

Secondly, there's nothing called good or bad idea. What fails an idea is absence of optimum planning and poor execution. Make sure you plan your road ahead into small achievable milestones.

Hope above should be of some help!!

One element of your question would concern me; if an incumbent sees your success, they could easily add your unique feature. If it really is just that simple, it seems like a risky project.
On the other hand, if you also have a lower margin business model that your competitors can't easily match then that risk is mitigated. For example, you launch with free base product and $1+/month for extra features whereas they are $10/month basic. It's hard for a company to come down in price like that (see Innovator's Dilemma).
Of course there are other factors, but that one stood out.