Create one complete copy
Open My List and choose Create backup. Copy all the text in the backup box, including its opening and closing braces. Keep it somewhere you can reach from the destination device.
Merge rather than replace
Paste the text into Restore backup and choose Merge backup. Valid titles already in the list are not added twice. Existing checkpoints remain in place when the same series appears in both lists, so review the episode you want to keep.
Read the result
The confirmation tells you what was added or skipped. An incomplete copy will produce an error without clearing your current list. The backup belongs to YassFlix; it is not an account export for another website or a file containing videos.