Make sure that you do not have any Leap repositories active.
When I migrated from Leap to Tumbleweed, I manually added the “tumbleweed/repo/oss/” and
“packman/suse/openSUSE_Tumbleweed/” (I have a Broadcom wireless chip that uses the kernel module from Packman) repositories and disabled all the others. Because the dup downloads are always pretty large, I use the “zypper dup -d” parameter to download everything before I start the dup proper.
Good Luck!