Both things reported here look fixed in 6.7: the retry succeeding but the job still showing as failed (PR #10129), and the EEXIST: file already exists error, which happened when a job got recreated after a crash (PR #10139). Curious if upgrading clears it for you all too.

I don't fully follow the mac-seed side of this, but a couple of things in the thread stand out. Tristis Oris's workaround looks like the practical unblock for now: removing the halted CR copy on the target host lets the migration go through, presumably because that replica VM is what collides on the mac-seed. Since you, KPS and Tristis Oris are all hitting the same DUPLICATE_MAC_SEED migrating into a replica target, this feels like something worth a GitHub issue on xen-orchestra with your XO commit, the exact steps, and whether a halted CR copy is present each time. It might also be worth a mention to @Team-XAPI-Network, since they'd know whether a CR replica is supposed to share its source's mac-seed. I could be wrong on the mechanism, so take that with a pinch of salt.

I quickly discussed with Xen developers and they are curious about your issue. Can you do a lspci -vvvv in your Dom0, first while you have still the option pci=realloc, and once without it, and see if we can spot any difference regarding your NIC. This might be helpful (you can provide both outputs at https://paste.vates.tech)