|Let me see if I have this correct.|
You have a bridged network between two buildings that is all one subnet. You replaced a router in one of the two buildings and whenever you plug the bridge cable into the new router, you get an IP conflict error.
I would start by tracing the dupe. Where is the device holding the IP? Where/what is the dupe? Is the device that's duping the new router? Is this IP statically assigned or DHCP? Once you know this information you can then work out from there.
Devices like routers, servers and printers should all have statically assigned IP's so as to avoid them ever having conflicts. I prefer to use IP's that are not in the DHCP scope for this. You can alternatively use reservations.
You will need to check your DHCP scope and all reservations/exclusions. Then check all statically assigned IP's starting with the router in the other building. Once you've done all the above, you can then statically assign an IP to the replacement router that isn't in the scope and doesn't conflict with any reservations/exclusions.
Just FYI, not capitalizing properly while you're typing makes it ever so much harder to read. Also, proper capitalization, spelling, grammar and punctuation shows professionalism and does not make you look like an illiterate buffoon.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.