I connected the "Isolator" valve(s) for tankless water heater, the cold water input and the hot water output. The water heater connections are 3/4" males connecting to the FIP of the isolator values, I put teflon tape on the thread, and tighten it very hard, it was so tight that I saw thread of copper being stripped off, but after everything is hooked up, I get small drops of water seeping through both the cold (input) and hot (output) water connections. What is the best way to seal the 3/4" flare with the 3/4" FIP connector? Even if the thread maybe partially ruined? Shall I use the pipe dope for gas pipe connection? Or use more teflon tape? Is there a better method? The slow drip is annoying. Thanks in advance.

Well if you saw a thread coming off the connector I think you know what's wrong. Once a connection is stripped its pretty much worthless and it can be both the male and female connector that needs replacing. If you want to give it one more try it won't cost you anything but unscrew your connection and put 4 or 5 full wraps of white teflon tape on the male threads, retighten but less than before. You could use pipe dope but it not just for gas, its for water pipes too. After your 5 wraps of teflon feel free to smear some dope on the tape if you want insurance. There is still a chance you striped those threads and will still need to replace both fitting on both pipes. That's the worst casr scenario.