I'm not a lawyer, but if you duplicate feature, or if your software is in the same field, or heavily inspired by the one you work on, they probably have ground

I'm not a lawyer, but if you duplicate feature, or if your software is in the same field, or heavily inspired by the one you work on, they probably have ground to have rights on it, regardless of where/how you wrote it... When I was an employee, I always added an addendum to my contract to make sure that there was a boundary between the 'work' stuff and my spare time github or other piece of code I might have done before. Specifically, I have almost 30 years of experience, and I do have a beepload of code that I don't want them to be able to claim. Also, if I contribute some of my code (it does happen) I first clear it with management, and I have a special header that specifies that it was my property, and is now /both/ our property as I give them an unlimited licence for it, as long as I also keep the present and future right to it. Saves potential headaches later on... if there is a fallout ;-) I doesn't need to be very complicated document, a single A4 is enough to make the situation clear, and I think any 'wise' employer will realize an experienced dev will come along with a toolbox. On the other hand, I would /never/ develop something that might lead to a conflict of interest, as you have. Bit of a hairball, that :-)