When a wish is being marked as a duplicate, it will be marked as declined and hidden from wishlist. We need to transfer the votes, if any, for the declined wish to the other wish that is being kept active in the wishlist.
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I imagine there should be consideration on which should be kept if the same user has voted for both wishes. I'd say don't transfer the vote for the closed one if it has no comment, and do if it does and the vote for the non-closed one doesn't. The question remains what to do if both have comment (and they differ). don't transfer the vote at all, given the two wishes may not describe the completely identical problem/request.
As a loose workaround when closing on W507, I tagged all the supporters to let them know and asked them to vote for the canonical wish. To echo the point from @Nardog above, another user (Paloi Sciurala) also stated:
Honestly, I prefer not to have my votes merged automatically, since I might not agree that the second wish is a duplicate of the first one. Speaking generally, not about this very wish.
I think we have just had less than a handful of duplicates in the past months, so I think the manual workaround is fine for now as long as we stay on top of the incoming wishes.
You're not echoing my point. I was talking strictly about when the wish being closed and the wish it's being merged into both already have a support vote by the same user. It's a no-brainer to transfer the votes by users who have not voted for the target wish.
Merging the votes but also pinging the users will also allow them to retract it from the target wish in case they disagree with the target wish. Wouldn't that be enough @FonAfon?