We ran into the same issue on a legacy EX4200. The only IP on the switch was the management IP. A subnet was not added when it was configured so it defualted to a /32. We needed to change this to a /24, but we ran into the same error everytime we tried to make the change. Since it was the only IP there was zero possibility of an actual overlap.
A reboot was required to resolve our issue, but here are a couple things I would try before going for the nuclear option:
1. do a 'commit full' - This is useful if you have a configuration that does not seem to be taking affect such as deleting an IP off an interface.
2. Verify there are no conflicting addresses. 'show configuration | display set | match inet | no-more'
3. delete the entire interface, not just the IP address. At least delete the family inet.
4. finally delete the family inet with its IP address and reboot to ensure that it takes affect.
We went through these steps (and more), but the only thing that worked for us was after step 4 we were able to add the IP address without the error. Thankfully this was a new install, so rebooting was an option for us since it did not have host on it yet.