Developing an existing idea does not consider stealing. Ideas are not exclusive to people. What you think about now, usually 100 people thought of it too. However, the difference is who will apply it first?
Sometimes, a small idea without development does not succeed + the success of projects depends on the application rather than the idea itself.
There are simple ideas, such as making greeting cards. Someone will design 20 cards, sell them from home, and copy them through an account on Instagram. While another one will rent stands in shops and hypermarkets, and with an agreement that a percentage will be for the hyper owner.
The same idea but the only difference is in applying it.