fix: handle multiple forks at same checkpoint

lib/network.py

@@ -835,13 +835,19 @@ class Network(util.DaemonThread):
                 if branch is not None:
                     if branch.check_header(interface.bad_header):
                         interface.print_error('joining chain', interface.bad)
+                        next_height = None
                     elif branch.parent().check_header(header):
                         interface.print_error('reorg', interface.bad, interface.tip)
                         interface.blockchain = branch.parent()
+                        next_height = None
                     else:
-                        # should not happen
+                        interface.print_error('checkpoint conflicts with existing fork', branch.path())
-                        raise BaseException('error')
+                        open(branch.path(), 'w+').close()
-                    next_height = None
+                        branch.save_header(interface.bad_header)
+                        interface.mode = 'catch_up'
+                        interface.blockchain = branch
+                        next_height = interface.bad + 1
+                        interface.blockchain.catch_up = interface.server
                 else:
                     bh = interface.blockchain.height()
                     next_height = None